Support viewport in content_shell.

Viewport preferences are overridden even with the --enable-viewport flag passed in.

BUG=343993

Review URL: https://codereview.chromium.org/166033008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252161 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252161