[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chrome/browser/printing.

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=dpapad@chromium.org

Bug: 874080
Change-Id: I23d9583ee267f5d88fb8c2e708fd7055d9f05951
Reviewed-on: https://chromium-review.googlesource.com/1191184Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589223}

chrome/browser/printing/printer_manager_dialog_linux.cc

@@ -13,7 +13,7 @@
 #include "base/process/kill.h"
 #include "base/process/launch.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 
 namespace {
 
@@ -53,7 +53,7 @@ bool OpenPrinterConfigDialog(const char* const* command) {
 // Detect the command based on the deskop environment and open the printer
 // manager dialog.
 void DetectAndOpenPrinterConfigDialog() {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   std::unique_ptr<base::Environment> env(base::Environment::Create());
 
   bool opened = false;