make installation of local extension file more easy

[Karl-Philipp Richter]

Currently installation of `.crx` files simply fails and one has to investigate information and the manual and figure out the "ExtensionInstallSources" section. `chromium-browser` should deal with that automatically and the user shouldn't be obliged to read the manual.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: chromium-browser 43.0.2357.130-0ubuntu0.15.04.1.1174
Uname: Linux 4.1.3-040103-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
DRM.card0.DP.1:
 edid-base64:
 dpms: On
 modes:
 enabled: disabled
 status: disconnected
DRM.card0.HDMI.A.1:
 edid-base64:
 dpms: On
 modes:
 enabled: disabled
 status: disconnected
DRM.card0.LVDS.1:
 edid-base64: AP///////wANrrUVAAAAAAIWAQOAIhN4CtH1k11ZkCYdUFQAAAABAQEBAQEBAQEBAQEBAQEB2B1W4lAAHzBELkYAWMIQAAAYpgAA/gBOMTU2QkdFLUxBMQogAAAA/gBDTU4KICAgICAgICAgAAAA/gBOMTU2QkdFLUxBMQogAJA=
 dpms: On
 modes: 1366x768
 enabled: enabled
 status: connected
DRM.card0.VGA.1:
 edid-base64: AP///////wAiZLg/AQEBARMUAQMKLx546txVo1lIniQRUFS/74BhRnFPgcCBQIGAlQ+pQLMAITmQMGIaJ0BosDYA2igRAAAcAAAA/ABIUDIyMgogICAgICAgAAAA/QA4SxhTEQAKICAgICAgAAAA/wAwMTlUUDNYWTAwMTUzAG8=
 dpms: On
 modes: 1680x1050 1600x1200 1280x1024 1280x1024 1440x900 1280x960 1152x864 1280x720 1024x768 1024x768 1024x768 1024x768 832x624 800x600 800x600 800x600 800x600 640x480 640x480 640x480 640x480 720x400
 enabled: enabled
 status: connected
Date: Sat Aug 1 13:04:44 2015
Desktop-Session:
 'ubuntu'
 '/etc/xdg/xdg-ubuntu:/usr/share/upstart/xdg:/etc/xdg:/usr/share/kubuntu-default-settings/kf5-settings'
 '/usr/share/ubuntu:/usr/share/gnome:/usr/local/share/:/usr/share/'
DetectedPlugins:

Env:
 'None'
 'None'
InstallationDate: Installed on 2015-06-08 (53 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
Load-Avg-1min: 3.32
Load-Processes-Running-Percent: 0.2%
MachineType: LENOVO 20221
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.1.3-040103-generic root=UUID=d665563a-2f33-4d06-bf9c-6838fe5ae3e9 ro rootflags=subvol=ubuntu-main-root
SourcePackage: chromium-browser
UpgradeStatus: Upgraded to vivid on 2015-06-09 (53 days ago)
dmi.bios.date: 07/12/2013
dmi.bios.vendor: LENOVO
dmi.bios.version: 71CN51WW(V1.21)
dmi.board.asset.tag: No Asset Tag
dmi.board.name: INVALID
dmi.board.vendor: LENOVO
dmi.board.version: 31900003WIN8 STD MLT
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Lenovo IdeaPad Z500 Touch
dmi.modalias: dmi:bvnLENOVO:bvr71CN51WW(V1.21):bd07/12/2013:svnLENOVO:pn20221:pvrLenovoIdeaPadZ500Touch:rvnLENOVO:rnINVALID:rvr31900003WIN8STDMLT:cvnLENOVO:ct10:cvrLenovoIdeaPadZ500Touch:
dmi.product.name: 20221
dmi.product.version: Lenovo IdeaPad Z500 Touch
dmi.sys.vendor: LENOVO
gconf-keys: /desktop/gnome/applications/browser/exec = b'firefox\n'/desktop/gnome/url-handlers/https/command = b'/opt/firefox/firefox %s\n'/desktop/gnome/url-handlers/https/enabled = b'true\n'/desktop/gnome/url-handlers/http/command = b'/opt/firefox/firefox %s\n'/desktop/gnome/url-handlers/http/enabled = b'true\n'/desktop/gnome/session/required_components/windowmanager = b''/apps/metacity/general/compositing_manager = b'true\n'/desktop/gnome/interface/icon_theme = b'ubuntu-mono-dark\n'/desktop/gnome/interface/gtk_theme = b'Ambiance\n'
modified.conffile..etc.default.chromium.browser: [deleted]