[android_webview] Make tests draw something other than white.

Setting FLAG_HARDWARE_ACCELERATED when we only support software rendering
leads to the test activity always rendering white.
Also, setting the root layout to MATCH_PARENT as that's a saner default.

BUG=None
TEST=AndroidWebViewTest
// android_webview-only, trybots are happy with the CL.
NOTRY=true

Review URL: https://codereview.chromium.org/44433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231029 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231029