add ext_float_blend feature and test Bug: chromium:930993 Change-Id: I8edbd01c5c9f1ed63243cc4a42f6de44c92db8bd Reviewed-on: https://chromium-review.googlesource.com/c/1481242 Commit-Queue: Shrek Shao <shrekshao@google.com> Reviewed-by: Jamie Madill <jmadill@google.com>