Clang update script: look for CMake in standard dir

This allows the script to find it even if it's not on PATH.

BUG=82385
NOTRY=true
TBR=rnk

Review URL: https://codereview.chromium.org/159983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250466 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250466