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::DoViewport(GLint x, GLint y, GLsizei width,
                                  GLsizei height) {
  state_.viewport_x = x;
  state_.viewport_y = y;
  state_.viewport_width = std::min(width, viewport_max_width_);
  state_.viewport_height = std::min(height, viewport_max_height_);
  glViewport(x, y, width, height);
}

void GLES2DecoderImpl::DoViewport(GLint x, GLint y, GLsizei width,
                                  GLsizei height) {
  state_.viewport_x = x;
  state_.viewport_y = y;
  state_.viewport_width = std::min(width, viewport_max_width_);
  state_.viewport_height = std::min(height, viewport_max_height_);
  glViewport(x, y, width, height);
}
