Don't use return value of compositeAndReadbackAsync so it can be void.

This pulls out the piece from https://codereview.chromium.org/266243002/
to stop using the return value of compositeAndReadbackAsync in order
to remove the return value and replace it with an ASSERT in
this: https://codereview.chromium.org/263253002/.

R=abarth
BUG=251960

Review URL: https://codereview.chromium.org/270463003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268958 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed