Revert "[TaskScheduler]: Migrate off of ScopedAllowWait in /android_webview"

This reverts commit bcc4146d.

Reason for revert: android.webkit.cts.WebViewStartupTest#testCookieManagerBlockingUiThread is consistently failing.

Bug: 905025

Original change's description:
> [TaskScheduler]: Migrate off of ScopedAllowWait in /android_webview
> 
> base::ThreadRestrictions::ScopedAllowWait is deprecated in favor of its more
> explicit counterpart.
> 
> It should have been replaced by :
>  * base::ScopedAllowBaseSyncPrimitivesForTesting in test files.
>  * base::ScopedAllowBaseSyncPrimitives in non-test files
>  * base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope when it's used on threads
>    that don't allow blocking
> The last one is strongly frowned upon but this CL aims to document existing
> behavior rather than address it. Owners are encouraged to follow-up by fixing
> unnecessary waits and more particularly unnecessary waits
> outside-blocking-scope.
> 
> Note: The non-for-testing versions require friend'ing in thread_restrictions.h
> but care was taken to add these friends ahead of git cl split (since it wasn't
> possible to do a line-by-line associated CL split).
> Refer to the top-level CL if necessary :
> https://chromium-review.googlesource.com/c/chromium/src/+/1288533
> 
> Please CQ if LGTY!
> 
> This CL was uploaded by git cl split.
> 
> R=​torne@chromium.org
> 
> Bug: 766678
> Change-Id: I30e57cb0d9a0b1a449a4227561e97ea330ff6a12
> Reviewed-on: https://chromium-review.googlesource.com/c/1325195
> Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Richard Coles <torne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#607674}

TBR=gab@chromium.org,torne@chromium.org,etiennep@chromium.org

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

Bug: 766678
Change-Id: If1ee0994f4387786662d7814617b297939b4dffb
Reviewed-on: https://chromium-review.googlesource.com/c/1337637Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608392}

android_webview/browser/aw_form_database_service.cc

@@ -84,7 +84,7 @@ bool AwFormDatabaseService::HasFormData() {
           base::IgnoreResult(&awds::GetCountOfValuesContainedBetween),
           autofill_data_, base::Time(), base::Time::Max(), this));
   {
-    base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope allow_wait;
+    base::ThreadRestrictions::ScopedAllowWait wait;
     has_form_data_completion_.Wait();
   }
   return has_form_data_result_;

android_webview/browser/cookie_manager.cc

@@ -247,7 +247,7 @@ void CookieManager::ExecCookieTaskSync(
                            base::WaitableEvent::InitialState::NOT_SIGNALED);
   ExecCookieTask(base::BindOnce(
       std::move(task), BoolCallbackAdapter(SignalEventClosure(&completion))));
-  base::ScopedAllowBaseSyncPrimitives wait;
+  base::ThreadRestrictions::ScopedAllowWait wait;
   completion.Wait();
 }
 
@@ -258,7 +258,7 @@ void CookieManager::ExecCookieTaskSync(
                            base::WaitableEvent::InitialState::NOT_SIGNALED);
   ExecCookieTask(base::BindOnce(
       std::move(task), IntCallbackAdapter(SignalEventClosure(&completion))));
-  base::ScopedAllowBaseSyncPrimitives wait;
+  base::ThreadRestrictions::ScopedAllowWait wait;
   completion.Wait();
 }
 
@@ -270,7 +270,7 @@ void CookieManager::ExecCookieTaskSync(
                            base::WaitableEvent::InitialState::NOT_SIGNALED);
   ExecCookieTask(
       base::BindOnce(std::move(task), SignalEventClosure(&completion)));
-  base::ScopedAllowBaseSyncPrimitives wait;
+  base::ThreadRestrictions::ScopedAllowWait wait;
   completion.Wait();
 }
 

base/threading/thread_restrictions.h

@@ -343,6 +343,7 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitives {
   friend class ::BrowserProcessImpl;
   friend class SimpleThread;
   friend class android::JavaHandlerThread;
+  friend class android_webview::AwFormDatabaseService;
   friend class android_webview::CookieManager;
   friend class base::GetAppOutputScopedAllowBaseSyncPrimitives;
   friend class base::StackSamplingProfiler;
@@ -393,21 +394,19 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitivesOutsideBlockingScope {
 
   // Allowed usage:
   friend class ::KeyStorageLinux;
-  friend class android_webview::
-      AwFormDatabaseService;  // http://crbug.com/904431
+  friend class Thread;
   friend class base::MessageLoopImpl;
   friend class content::ScopedAllowWaitForDebugURL;
   friend class content::SynchronousCompositor;
   friend class content::SynchronousCompositorHost;
   friend class content::SynchronousCompositorSyncCallBridge;
   friend class mojo::SyncCallRestrictions;
-  friend class Thread;
   friend class viz::HostGpuMemoryBufferManager;
 
   // Usage that should be fixed:
   friend class ::chromeos::BlockingMethodCaller;  // http://crbug.com/125360
-  friend class cc::CompletionEvent;               // http://crbug.com/902653
-  friend class cc::SingleThreadTaskGraphRunner;   // http://crbug.com/902823
+  friend class cc::CompletionEvent;              // http://crbug.com/902653
+  friend class cc::SingleThreadTaskGraphRunner;  // http://crbug.com/902823
   friend class content::
       BrowserGpuChannelHostFactory;                 // http://crbug.com/125248
   friend class content::CategorizedWorkerPool;      // http://crbug.com/902823