Build the latest release of Chrome 28

VERSION now specifies a Chrome release instead of a Chromium trunk
revision. script/update now generates vendor/chromium/.gclient on demand
based on the specified release.

By building Chrome releases we can hopefully avoid bugs that Google
never ships to customers, at the expense of some modernity.
Update custom_deps to match latest recommendations

These came from http://dev.chromium.org/developers/how-tos/get-the-code
Update ResourceUpdate patch to match Chrome 28
Remove v8::AssertNoGCScope patch

This isn't needed for Chrome 28. We'll probably need to re-add it when
we update to Chrome 29.
Merge pull request #19 from brightray/chrome-28

Build the latest release of Chrome 28