[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chromecast.

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=servolk@chromium.org

Bug: 874080
Change-Id: I57c774dacb924224a77f31a9644242a6931d52cf
Reviewed-on: https://chromium-review.googlesource.com/1191235Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586390}

chromecast/crash/linux/dummy_minidump_generator.cc

@@ -6,7 +6,7 @@
 
 #include "base/files/file_util.h"
 #include "base/logging.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 
 namespace chromecast {
 
@@ -16,7 +16,7 @@ DummyMinidumpGenerator::DummyMinidumpGenerator(
 }
 
 bool DummyMinidumpGenerator::Generate(const std::string& minidump_path) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   // Return false if the file does not exist.
   if (!base::PathExists(base::FilePath(existing_minidump_path_))) {