Android: Support a prebuilt WebView.

When building using the Android platform build system, if the current
product sets PRODUCT_PREBUILT_WEBVIEWCHROMIUM, do not attempt to build
the WebView from source.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/66343006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234848 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234848