WebView: avoid library name clash when building standalone.
The standalone libwebviewchromium.so "clashes" with the one built using
android_webview_build==1; rename it in the standalone case so that if
you end up with both available, the correct one is used.
product_prefix is used because this ensures that the existing logic to
use product_extension to make component builds work is not disrupted.
Tested change with component and non-component builds.
BUG=
Review URL: https://codereview.chromium.org/
37373002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230485 0039d316-1c4b-4281-b951-d872f2087c98