[Extensions] Simplify crash-demo.html
[Extension] Add browsertest for ExtensionProcess crash.

Currently, when the ExtensionProcess crashes nothing should happen
to the BrowserProcess. The RenderProcess, however, should shutdown.
This means that if the page causes a crash on EP, RP shouldn't be
able to process javascript anymore, thus bein unable to update the
window title on BP.
Merge pull request #1149 from jeez/crash-browsertest

[Extension] Add browsertest for ExtensionProcess crash.