[Paint Preview] Clarify thread behaviors

Clairfy the correct thread behaviors for PaintPreviewClient and
PaintPreviewBaseService.

Largely just a documentation change.

Change-Id: Ia1f9cadc8420a80610b3fd8ddb17d77bae3b12d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090198Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747780}

components/paint_preview/browser/paint_preview_base_service.cc

@@ -20,6 +20,7 @@
 #include "components/paint_preview/browser/paint_preview_client.h"
 #include "components/paint_preview/browser/paint_preview_compositor_service_impl.h"
 #include "components/paint_preview/common/mojom/paint_preview_recorder.mojom.h"
+#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/web_contents.h"
 #include "ui/gfx/geometry/rect.h"
 
@@ -73,6 +74,7 @@ void PaintPreviewBaseService::CapturePaintPreview(
     const base::FilePath& root_dir,
     gfx::Rect clip_rect,
     OnCapturedCallback callback) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   if (policy_ && !policy_->SupportedForContents(web_contents)) {
     std::move(callback).Run(kContentUnsupported, nullptr);
     return;

components/paint_preview/browser/paint_preview_base_service.h

@@ -81,6 +81,11 @@ class PaintPreviewBaseService : public KeyedService {
       const DirectoryKey& key,
       OnReadProtoCallback on_read_proto_callback);
 
+  // Captures need to run on the Browser UI thread! Captures may involve child
+  // frames so the PaintPreviewClient (WebContentsObserver) must be stored as
+  // WebContentsUserData which is not thread safe and must only be accessible
+  // from a specific sequence i.e. the UI thread.
+  //
   // The following methods both capture a Paint Preview; however, their behavior
   // and intended use is different. The first method is intended for capturing
   // full page contents. Generally, this is what you should be using for most

components/paint_preview/browser/paint_preview_client.cc

@@ -193,8 +193,6 @@ void PaintPreviewClient::RenderFrameDeleted(
 
 PaintPreviewClient::CreateResult PaintPreviewClient::CreateFileHandle(
     const base::FilePath& path) {
-  base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
-                                                base::BlockingType::MAY_BLOCK);
   base::File file(path,
                   base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
   return CreateResult(std::move(file), file.error_details());
@@ -215,6 +213,7 @@ 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();

components/paint_preview/browser/paint_preview_client.h

@@ -27,6 +27,9 @@ namespace paint_preview {
 // Client responsible for making requests to the mojom::PaintPreviewService. A
 // client coordinates between multiple frames and handles capture and
 // aggreagation of data from both the main frame and subframes.
+//
+// Should be created and accessed from the UI thread as WebContentsUserData
+// requires this behavior.
 class PaintPreviewClient
     : public content::WebContentsUserData<PaintPreviewClient>,
       public content::WebContentsObserver {
@@ -57,6 +60,8 @@ class PaintPreviewClient
 
   ~PaintPreviewClient() override;
 
+  // IMPORTANT: The Capture* methods must be called on the UI thread!
+
   // 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