Default revision to 'HEAD' if None
Default revision to 'HEAD' if None

Otherwise we see "src@None,src/third_party/WebKit@HEAD"

BUG=261741

Review URL: https://codereview.chromium.org/362843002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@280904 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build