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::HandleShaderSourceBucket(
  uint32 immediate_data_size, const cmds::ShaderSourceBucket& c) {
  Bucket* bucket = GetBucket(c.data_bucket_id);
  if (!bucket || bucket->size() == 0) {
    return error::kInvalidArguments;
  }
  return ShaderSourceHelper(
      c.shader, bucket->GetDataAs<const char*>(0, bucket->size() - 1),
      bucket->size() - 1);
}

error::Error GLES2DecoderImpl::HandleShaderSourceBucket(
  uint32 immediate_data_size, const cmds::ShaderSourceBucket& c) {
  Bucket* bucket = GetBucket(c.data_bucket_id);
  if (!bucket || bucket->size() == 0) {
    return error::kInvalidArguments;
  }
  return ShaderSourceHelper(
      c.shader, bucket->GetDataAs<const char*>(0, bucket->size() - 1),
      bucket->size() - 1);
}
