[Extensions] Fix Zombie extension process on Linux after a ctrl+c

On Linux we are currently leaving the extension process alive
even after its parent, the BrowserProcess, was killed with ctrl+c
(^C, or SIGTERM).

We still haven't found a more chromium-ish way of doing this,
so let's stick to prctl.h for now.
[Extensions] Change switch to --disable-extension--process

This will enable the ExtensionProcess (EP) by default for all
platforms, but shouldn't affect Android. On this platform,
XWalkExtensionService::RegisterExtension is called directly,
which makes extensions to be registered on the in_process
XWalkExtensionServer.
[Extensions][Android] Disable ExtensionProcess by default
[Extensions][Tests] Make ExternalExtension tests use the ExtensionProcess
[Extensions] Use base::GetNativeLibraryName in RegisterExternalExtensionsInDirectory

This will avoid us having problems with .so vs .dll (Linux vs Windows).
[Extensions][Windows] Disable ExtensionProcess for Windows
Merge pull request #676 from jeez/ep-final

[Extensions] Make ExtensionProcess enabled by default