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