Search for the source root rather than checkout root in clang_format.py.
Search for the source root rather than checkout root in clang_format.py.

Use the source root (src/ in Chromium) rather than the checkout root as
the path from which to locate the clang-format binaries.

webrtc uses the Chromium build/tools infrastructure but doesn't have
src/ as a source root path. This change allows us to continue to use
'git cl format' in a standalone webrtc checkout (which would be nice.)

BUG=240309
TESTED='git cl format' works on a trivial test change in both a
Chromium and webrtc checkout.

Review URL: https://codereview.chromium.org/135653014

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@248651 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools