[android_webview] Clean up AwContents constructors.

We have way too many AwContents constructors.

BUG=None
TEST=AndroidWebViewTest
Android-only change, trybots happy.
NOTRY=true

Review URL: https://codereview.chromium.org/40663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234799 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234799