ExtensionSystem cleanup part 3

Fix most users of ExtensionSystemFactory::GetInstance() to use ExtensionsBrowserClient::Get()->GetExtensionSystemFactory(). Clean up extension_system_factory.h includes.

Follows https://codereview.chromium.org/137753016/

BUG=337707

Review URL: https://codereview.chromium.org/148083014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251284 0039d316-1c4b-4281-b951-d872f2087c98
Revert "linux-aura/webrtc: Disable a couple of tests failing on the bots."

Flakiness should be fixed.  Renabling tests.

BUG=335916,341265
TEST=trybots

Review URL: https://codereview.chromium.org/145093006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251285 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251285