Moved the ManagedBookmarksPolicyHandler to a shared file.

This change allows reusing that policy handler on iOS too. The strings used
by this handler are also included on iOS builds now.

BUG=349983

Review URL: https://codereview.chromium.org/199233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256922 0039d316-1c4b-4281-b951-d872f2087c98
Update common.gypi to set clang=1 for all iOS builds.

All iOS builds now use clang, regardless of which gyp generator is used, so
change common.gypi to reflect that.

NOTRY=True
BUG=352167

Review URL: https://codereview.chromium.org/197283006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256923 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256923