[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /content/browser/web_contents/web_contents_android.cc.

This CL was created by replacing calls to AssertBlockingAllowed()
with instantiations of ScopedBlockingCall(MAY_BLOCK).
I kindly ask the reviewer to make sure of the following:
  - ScopedBlockingCall is instantiated in a scope with minimal CPU usage.
    If this is not the case, ScopedBlockingCall should be instantiated
    closer to the blocking call. See scoped_blocking_call.h for more
    info. Please let me know when/where the blocking call happens if this needs
    to be changed.
  - Parameter |blocking_type| matches expectation (MAY_BLOCK/WILL_BLOCK). See
    BlockingType for more info. While I assumed MAY_BLOCK by default, that might
    not be the best fit if we know that this callsite is guaranteed to block.
  - The ScopedBlockingCall's scope covers the entirety of the blocking operation
    previously asserted against by the AssertBlockingAllowed().

This CL was uploaded by git cl split.

R=tedchoc@chromium.org

Bug: 874080
Change-Id: I83ad9ab928cecc0e1442f76f6036e185d1d5f29a
Reviewed-on: https://chromium-review.googlesource.com/1191785
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587113}

content/browser/web_contents/web_contents_android.cc

@@ -20,6 +20,7 @@
 #include "base/logging.h"
 #include "base/metrics/user_metrics.h"
 #include "base/task/post_task.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "content/browser/accessibility/browser_accessibility_android.h"
 #include "content/browser/accessibility/browser_accessibility_manager_android.h"
 #include "content/browser/android/java/gin_java_bridge_dispatcher_host.h"
@@ -142,7 +143,7 @@ void AXTreeSnapshotCallback(const ScopedJavaGlobalRef<jobject>& callback,
 
 std::string CompressAndSaveBitmap(const std::string& dir,
                                   const SkBitmap& bitmap) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::WILL_BLOCK);
 
   std::vector<unsigned char> data;
   if (!gfx::JPEGCodec::Encode(bitmap, 85, &data)) {