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: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@248651 0039d316-1c4b-4281-b951-d872f2087c98