sync-webkit-git: handle DEPS specifying nonexistent revision
sync-webkit-git: handle DEPS specifying nonexistent revision

Suppose WebKit trunk's recent history is commits r1, r5, and r10.
Then suppose our DEPS file asks for r8.  We should then use r5 as
that contains the same code as r8.

Note that if recent history is as above but lacking r10, we still
die, because it's possible that r8 exists on trunk but just hasn't
made it into git.

Review URL: http://codereview.chromium.org/546090

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@36655 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser