Use TaskScheduler instead of WorkerPool in navigation_entry_screenshot_manager.cc.

content/browser/frame_host/navigation_entry_screenshot_manager.cc

Index: content/browser/frame_host/navigation_entry_screenshot_manager.cc
diff --git a/content/browser/frame_host/navigation_entry_screenshot_manager.cc b/content/browser/frame_host/navigation_entry_screenshot_manager.cc
index 44bc2a03faa361a7d92d3320c7ea729885e526cd..01f90564b60794239da94522326a7aeff68fead5 100644
--- a/content/browser/frame_host/navigation_entry_screenshot_manager.cc
+++ b/content/browser/frame_host/navigation_entry_screenshot_manager.cc
@@ -6,7 +6,7 @@
 
 #include "base/command_line.h"
 #include "base/macros.h"
-#include "base/threading/worker_pool.h"
+#include "base/task_scheduler/post_task.h"
 #include "content/browser/frame_host/navigation_controller_impl.h"
 #include "content/browser/frame_host/navigation_entry_impl.h"
 #include "content/browser/renderer_host/render_view_host_impl.h"
@@ -32,14 +32,10 @@ class ScreenshotData : public base::RefCountedThreadSafe<ScreenshotData> {
   }
 
   void EncodeScreenshot(const SkBitmap& bitmap, base::Closure callback) {
-    if (!base::WorkerPool::PostTaskAndReply(FROM_HERE,
-            base::Bind(&ScreenshotData::EncodeOnWorker,
-                       this,
-                       bitmap),
-            callback,
-            true)) {

[alexmos, 2017/01/03 19:19:06]

Is there's an equivalent to task_is_slow in TaskScheduler?  I'm not too familiar
with how important this is -- is it ok to lose it?

[fdoray, 2017/01/04 00:27:09]

|task_is_slow| is ignored by WorkerPool and developers told us that they don't
like the fact that there are no clear guidelines to determine whether a task is
slow.

There is no equivalent to |task_is_slow| in TaskScheduler. However, there is a
MayBlock() trait for tasks that block (e.g. synchronously read a file from
disk).  When a MayBlock() task takes a long time to run, TaskScheduler may
decide to create a new thread (note that TaskScheduler dynamically determines
whether a task takes a long time to run).

FWICT, ScreenshotData::EncodeOnWorker is CPU-bound. Therefore, it should not be
posted with MayBlock().

[alexmos, 2017/01/04 01:00:08]

At least on Windows, it seems to be used to add WT_EXECUTELONGFUNCTION to
QueueUserWorkItem flags, see 
https://cs.chromium.org/chromium/src/base/threading/worker_pool_win.cc?l=46

Losing that was mostly why I was curious about this.
Acknowledged.

[fdoray, 2017/01/04 02:13:15]

Our experiments show that this flag is ignored on newer versions of Windows (it
was used on XP).

-      callback.Run();
-    }
+    base::PostTaskWithTraitsAndReply(
+        FROM_HERE, base::TaskTraits().WithShutdownBehavior(
+                       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN),
+        base::Bind(&ScreenshotData::EncodeOnWorker, this, bitmap), callback);
   }
 
   scoped_refptr<base::RefCountedBytes> data() const { return data_; }