Revert "[Paint Preview] Change PaintPreviewClient callback thread"

This reverts commit bc36a7c5.

Reason for revert: crbug/1058886

Seems to be causing some instability. Reverting for now...

Original change's description:
> [Paint Preview] Change PaintPreviewClient callback thread
>
> The callbacks passed to PaintPreviewClient were getting run on an
> arbitrary thread via PostTask. This is a problem as they should be run
> on the calling thread such that WeakPtrs bound to them are valid.
>
> Fix
> - Make the callbacks run on the same thread that called capture.
> - Explicitly run CapturePaintPreviewInternal on the UI thread.
>
> Follow-up
> - Make a dedicated SequencedTaskRunner for executing captures.
>
> Bug: 1058079
> Change-Id: I1dfa969de613112b4bd1650051e394094dffda5e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083567
> Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
> Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#746517}

TBR=mahmoudi@chromium.org,ckitagawa@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1058079,1058886
Change-Id: I714946368a528d6b2def20813bcf5b073aa2d2d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090292Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747389}

components/paint_preview/browser/paint_preview_client.cc

@@ -135,13 +135,8 @@ void PaintPreviewClient::CapturePaintPreview(
   document_data.root_url = render_frame_host->GetLastCommittedURL();
   document_data.source_id =
       ukm::GetSourceIdForWebContentsDocument(web_contents());
-  document_data.called_on_thread = base::SequencedTaskRunnerHandle::Get();
   all_document_data_.insert({params.document_guid, std::move(document_data)});
-  base::PostTask(
-      FROM_HERE, {content::BrowserThread::UI},
-      base::BindOnce(&PaintPreviewClient::CapturePaintPreviewInternal,
-                     weak_ptr_factory_.GetWeakPtr(), params,
-                     render_frame_host));
+  CapturePaintPreviewInternal(params, render_frame_host);
 }
 
 void PaintPreviewClient::CaptureSubframePaintPreview(
@@ -152,11 +147,7 @@ void PaintPreviewClient::CaptureSubframePaintPreview(
   params.document_guid = guid;
   params.clip_rect = rect;
   params.is_main_frame = false;
-  base::PostTask(
-      FROM_HERE, {content::BrowserThread::UI},
-      base::BindOnce(&PaintPreviewClient::CapturePaintPreviewInternal,
-                     weak_ptr_factory_.GetWeakPtr(), params,
-                     render_subframe_host));
+  CapturePaintPreviewInternal(params, render_subframe_host);
 }
 
 void PaintPreviewClient::RenderFrameDeleted(
@@ -224,7 +215,6 @@ mojom::PaintPreviewCaptureParamsPtr PaintPreviewClient::CreateMojoParams(
 void PaintPreviewClient::CapturePaintPreviewInternal(
     const PaintPreviewParams& params,
     content::RenderFrameHost* render_frame_host) {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   // Use a frame's embedding token as its GUID. Note that we create a GUID for
   // the main frame so that we can treat it the same as other frames.
   auto token = render_frame_host->GetEmbeddingToken();
@@ -270,7 +260,9 @@ void PaintPreviewClient::RequestCaptureOnUIThread(
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   auto* document_data = &all_document_data_[params.document_guid];
   if (result.error != base::File::FILE_OK) {
-    document_data->called_on_thread->PostTask(
+    // Don't block up the UI thread and answer the callback on a different
+    // thread.
+    base::PostTask(
         FROM_HERE,
         base::BindOnce(std::move(document_data->callback), params.document_guid,
                        mojom::PaintPreviewStatus::kFileCreationError, nullptr));
@@ -393,7 +385,7 @@ void PaintPreviewClient::OnFinished(base::UnguessableToken guid,
     // At a minimum one frame was captured successfully, it is up to the
     // caller to decide if a partial success is acceptable based on what is
     // contained in the proto.
-    document_data->called_on_thread->PostTask(
+    base::PostTask(
         FROM_HERE,
         base::BindOnce(std::move(document_data->callback), guid,
                        document_data->had_error
@@ -402,8 +394,8 @@ void PaintPreviewClient::OnFinished(base::UnguessableToken guid,
                        std::move(document_data->proto)));
   } else {
     // A proto could not be created indicating all frames failed to capture.
-    document_data->called_on_thread->PostTask(
-        FROM_HERE, base::BindOnce(std::move(document_data->callback), guid,
+    base::PostTask(FROM_HERE,
+                   base::BindOnce(std::move(document_data->callback), guid,
                                   mojom::PaintPreviewStatus::kFailed, nullptr));
   }
   all_document_data_.erase(guid);

components/paint_preview/browser/paint_preview_client.h

@@ -57,9 +57,6 @@ class PaintPreviewClient
 
   ~PaintPreviewClient() override;
 
-  // These methods do not need to be run on the UI thread, but do need to be run
-  // from a SequencedTaskRunner.
-
   // Captures a paint preview corresponding to the content of
   // |render_frame_host|. This will work for capturing entire documents if
   // passed the main frame or for just a specific subframe depending on
@@ -107,7 +104,6 @@ class PaintPreviewClient
 
     // Callback that is invoked on completion of data.
     PaintPreviewCallback callback;
-    scoped_refptr<base::SequencedTaskRunner> called_on_thread;
 
     // All the render frames that are still required.
     base::flat_set<base::UnguessableToken> awaiting_subframes;
@@ -152,7 +148,7 @@ class PaintPreviewClient
       base::File file);
 
   // Sets up for a capture of a frame on |render_frame_host| according to
-  // |params|. This should be called from the UI thread.
+  // |params|.
   void CapturePaintPreviewInternal(const PaintPreviewParams& params,
                                    content::RenderFrameHost* render_frame_host);