Changed two assertions to explicit tests, and added an error code to
those checked during ReadPixels. These changes are needed to avoid
having ANGLE assert in debug builds when running WebGL test cases
which provoke a GPU reset.

BUG=none
TEST=slow-shader-example and lots-of-polys-example in WebGL test suite
Review URL: http://codereview.appspot.com/4684042

git-svn-id: https://angleproject.googlecode.com/svn/trunk@703 736b8ea6-26fd-11df-bfd4-992fa37f6226
2 files changed