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
error::Error GLES2DecoderImpl::HandleDrawElementsInstancedANGLE(
    uint32 immediate_data_size, const cmds::DrawElementsInstancedANGLE& c) {
  if (!features().angle_instanced_arrays) {
    LOCAL_SET_GL_ERROR(
        GL_INVALID_OPERATION,
        "glDrawElementsInstancedANGLE", "function not available");
    return error::kNoError;
  }
  return DoDrawElements("glDrawElementsInstancedANGLE",
                        true,
                        static_cast<GLenum>(c.mode),
                        static_cast<GLsizei>(c.count),
                        static_cast<GLenum>(c.type),
                        static_cast<int32>(c.index_offset),
                        static_cast<GLsizei>(c.primcount));
}

error::Error GLES2DecoderImpl::HandleDrawElementsInstancedANGLE(
    uint32 immediate_data_size, const cmds::DrawElementsInstancedANGLE& c) {
  if (!features().angle_instanced_arrays) {
    LOCAL_SET_GL_ERROR(
        GL_INVALID_OPERATION,
        "glDrawElementsInstancedANGLE", "function not available");
    return error::kNoError;
  }
  return DoDrawElements("glDrawElementsInstancedANGLE",
                        true,
                        static_cast<GLenum>(c.mode),
                        static_cast<GLsizei>(c.count),
                        static_cast<GLenum>(c.type),
                        static_cast<int32>(c.index_offset),
                        static_cast<GLsizei>(c.primcount));
}
