[base] Make PostTask destination explicit in /content/renderer

Task APIs v3 will all have an explicit destination. In the past
"default traits" meant ThreadPool. These CLs are thus a no-op
except for the few callers that clearly wanted to post a QuitClosure()
on the current thread (posting a QuitClosure() to a parallel task will
merely bounce it back to the current thread anyways). Those few callers
were migrated to ThreadTaskRunnerHandle::Get().

These changes are thus required to phase out task APIs v2 (post_task.h)
Design doc: https://docs.google.com/document/d/1tssusPykvx3g0gvbvU4HxGyn3MjJlIylnsH13-Tv6s4/edit

This specific change is a subset of
https://chromium-review.googlesource.com/c/chromium/src/+/2216733

This CL was uploaded by git cl split.

R=dcheng@chromium.org

Bug: 1026641
Change-Id: Ifcae70690c28466e17c93ea1b122dc8c48f205f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2217208
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772637}

content/renderer/render_view_browsertest.cc

@@ -18,7 +18,6 @@
 #include "base/stl_util.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/task/post_task.h"
 #include "base/test/bind_test_util.h"
 #include "base/test/gmock_callback_support.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ -1547,7 +1546,8 @@ TEST_F(RenderViewImplTest, EditContextGetLayoutBoundsAndInputPanelPolicy) {
   // This RunLoop is waiting for EditContext to be created and layout bounds
   // to be updated in the EditContext.
   base::RunLoop run_loop;
-  base::PostTask(FROM_HERE, run_loop.QuitClosure());
+  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
+                                                run_loop.QuitClosure());
   run_loop.Run();
 
   // Update the IME status and verify if our IME backend sends an IPC message
@@ -1591,7 +1591,8 @@ TEST_F(RenderViewImplTest, EditContextGetLayoutBoundsWithFloatingValues) {
   // This RunLoop is waiting for EditContext to be created and layout bounds
   // to be updated in the EditContext.
   base::RunLoop run_loop;
-  base::PostTask(FROM_HERE, run_loop.QuitClosure());
+  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
+                                                run_loop.QuitClosure());
   run_loop.Run();
   // Update the IME status and verify if our IME backend sends an IPC message
   // to notify layout bounds of the EditContext.
@@ -1629,7 +1630,8 @@ TEST_F(RenderViewImplTest, ActiveElementGetLayoutBounds) {
   ExecuteJavaScriptForTests("document.getElementById('test').focus();");
   // This RunLoop is waiting for focus to be processed for the active element.
   base::RunLoop run_loop;
-  base::PostTask(FROM_HERE, run_loop.QuitClosure());
+  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
+                                                run_loop.QuitClosure());
   run_loop.Run();
   // Update the IME status and verify if our IME backend sends an IPC message
   // to notify layout bounds of the EditContext.