[Android WebView] Enable the Blink quirk for merging contents of meta viewport tags

This enables support for mobile frameworks that dynamically add several meta
viewport tags -- instead of discarding the previous tag when processing the new
one, we merge their values.

BUG=306548

Review URL: https://codereview.chromium.org/26144003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230129 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230129