[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /components/crash.

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=mark@chromium.org

Bug: 874080
Change-Id: I85dff1e7321fcebb76a6fa2ff98046bd78377154
Reviewed-on: https://chromium-review.googlesource.com/1191803Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586701}

components/crash/content/browser/crash_handler_host_linux.cc

@@ -31,8 +31,8 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/task/post_task.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "base/threading/thread.h"
-#include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_thread.h"
@@ -410,7 +410,7 @@ void CrashHandlerHostLinux::FindCrashingThreadAndDump(
 void CrashHandlerHostLinux::WriteDumpFile(BreakpadInfo* info,
                                           std::unique_ptr<char[]> crash_context,
                                           pid_t crashing_pid) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   // Set |info->distro| here because base::GetLinuxDistro() needs to run on a
   // blocking sequence.