Framebuffer clear() needs to consider the situation some draw buffers are disabled.

This is when we expose DrawBuffers extension.

BUG=376951
TEST=the attached test case, webgl conformance
R=kbr@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/315283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275338 0039d316-1c4b-4281-b951-d872f2087c98
void GLES2DecoderImpl::DoSampleCoverage(GLclampf value, GLboolean invert) {
  state_.sample_coverage_value = std::min(1.0f, std::max(0.0f, value));
  state_.sample_coverage_invert = (invert != 0);
  glSampleCoverage(state_.sample_coverage_value, invert);
}

void GLES2DecoderImpl::DoSampleCoverage(GLclampf value, GLboolean invert) {
  state_.sample_coverage_value = std::min(1.0f, std::max(0.0f, value));
  state_.sample_coverage_invert = (invert != 0);
  glSampleCoverage(state_.sample_coverage_value, invert);
}
