Exclude about:help strings from Android builds.

Chrome for Android does not use chrome://help so we should not include
the strings for it.

BUG=326023
R=newt@chromium.org

Review URL: https://codereview.chromium.org/142573005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247789 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247789