Add support for user-defined source root directory.

This fixes a problem on the Linux Module builders where tests
 built as sub-projects (gyp r814) can't find the source root
because they're more than 2 levels deep
(http://build.chromium.org/buildbot/waterfall/builders/Modules%20Linux%20(dbg)/builds/24363/steps/base_unittests/logs/stdio).

Review URL: http://codereview.chromium.org/1690004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45260 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed