[blink] Add DCHECKs to enforce immutability of image after all data

 received.

This is already obeyed in the code, but not enforced. Add DCHECKs to
 make it explicit.

Bug: 1142834
Change-Id: I08c6ec08563c662fb5f98be6c965721655c87fbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535611
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827781}

third_party/blink/renderer/platform/graphics/bitmap_image_test.cc

@@ -276,12 +276,6 @@ TEST_F(BitmapImageTest, isAllDataReceived) {
 
   image->SetData(image_data, true);
   EXPECT_TRUE(image->IsAllDataReceived());
-
-  image->SetData(SharedBuffer::Create("data", sizeof("data")), false);
-  EXPECT_FALSE(image->IsAllDataReceived());
-
-  image->SetData(image_data, true);
-  EXPECT_TRUE(image->IsAllDataReceived());
 }
 
 TEST_F(BitmapImageTest, noColorProfile) {

third_party/blink/renderer/platform/graphics/deferred_image_decoder.cc

@@ -266,6 +266,8 @@ void DeferredImageDecoder::SetData(scoped_refptr<SharedBuffer> data,
 void DeferredImageDecoder::SetDataInternal(scoped_refptr<SharedBuffer> data,
                                            bool all_data_received,
                                            bool push_data_to_decoder) {
+  // Once all the data has been received, the image should not change.
+  DCHECK(!all_data_received_);
   if (metadata_decoder_) {
     all_data_received_ = all_data_received;
     if (push_data_to_decoder)