Remove the webkit_revision variable when patch doesn't specify a revision.
Remove the webkit_revision variable when patch doesn't specify a revision.

This is a follow up to https://codereview.chromium.org/12221058/ .
Without this, we end up setting the webkit revision to the chromium revision.

TBR=maruel@chromium.org
Review URL: https://codereview.chromium.org/12321023

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@183632 0039d316-1c4b-4281-b951-d872f2087c98

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