Fix Clear validation assert for default FBOs The validation was iterating over maxDrawBuffers attachments when default framebuffers only have one attachment. Use the framebuffer's drawBufferCount instead. Also adds a regression test in the form of a WebGLComptibility test for glClearBuffer with the default framebuffer. BUG=angleproject:2091 Change-Id: I07ee524db1fcb8a99dab4043248c0885100fd216