Android WebView: allow tests to not start AwBrowserProcess.

Add a way for tests to specify that they don't want to start
AwBrowserProcess before running the test, so that we can test things
which need to be able to run before we know which thread will become the
main thread. Move the start call in the shell app to the activity class
instead of the application class to make this work.

BUG=304813
R=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/68833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234237 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234237