Support userAgent overriding for guests.

Following methods are implemented:
<webview>.setUserAgentOverride(string);
<webview>.getUserAgent();
bool <webview>.isUserAgentOverridden();

Usage:
webviewElement.setUserAgentOverride('...');
webviewElement.setAttribute('src', '...);


BUG=275949
Test=Added WebViewTest.UserAgent, for use case, see Usage above.

Review URL: https://codereview.chromium.org/19267003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225301 0039d316-1c4b-4281-b951-d872f2087c98
Remove VolumeManager.mountDrive().

Now, VolumeManager has Drive's volume (if it is enabled) when it is ready, and
the mounting state is managed by C++ backend.
So, Files.app doesn't need to request mount the Drive from JS.

BUG=268817
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.

Review URL: https://codereview.chromium.org/23440065

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225302 0039d316-1c4b-4281-b951-d872f2087c98
Added NetworkInterface::network_prefix.

BUG=295997
NOTRY=true

Review URL: https://codereview.chromium.org/23726043

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225303 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225302
Adds code primarily to AvailabilityFinder and APISchemaGraph
for the purpose of finding object-level availability.

The general process:

AvailabilityFinder uses a HostFileSystemIterator to loop
over file systems. It adds availability data for an API to
an APISchemaGraph at each iteration. This Graph is returned
to the caller, and can be used to look up availability
information.

Note that this code is not actually called from within
APIDataSource, and no data is being given to the templates
at this time.

BUG=233982
NOTRY=true

Review URL: https://codereview.chromium.org/24031005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225304 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225304