Add target for building Chromium tests for WebRTC.

WebRTC is running the content_browsertests test
for ToT WebRTC revisions to track regressions on an
early stage. In order to save compile time and
build artifacts, we want to build only what's needed
(i.e. not the All target).
In https://codereview.chromium.org/27622005/ an
attempt was made to make content_browsertests_apk
to be enough of target to build (that CL made it also
build android_tools). Since that approach was having
bad sideffects, this helper target should give us
everything needed for our build.

NOTRY=True
TEST=local compile, verifying the apk and host_forwarder is built.
BUG=305749

Review URL: https://codereview.chromium.org/43463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232956 0039d316-1c4b-4281-b951-d872f2087c98
Added browser tests for adb_client_socket.cc and android_device.cc.

BUG=308535

Review URL: https://codereview.chromium.org/43793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232957 0039d316-1c4b-4281-b951-d872f2087c98
Media Galleries: Use the same last directory entry as file_system_api.

Review URL: https://codereview.chromium.org/46973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232958 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232958