[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chromeos/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=thestig@chromium.org

Bug: 874080
Change-Id: Ic3048f1874bd6d575a061c5f9f773df4850d6d32
Reviewed-on: https://chromium-review.googlesource.com/1191099Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587670}

chromeos/printing/ppd_cache.cc

@@ -17,8 +17,8 @@
 #include "base/synchronization/lock.h"
 #include "base/task/post_task.h"
 #include "base/task_runner_util.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "base/threading/sequenced_task_runner_handle.h"
-#include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "chromeos/printing/printing_constants.h"
@@ -47,7 +47,7 @@ void MaybeCreateCache(const base::FilePath& base_dir) {
 // allows I/O.
 PpdCache::FindResult FindImpl(const base::FilePath& cache_dir,
                               const std::string& key) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   PpdCache::FindResult result;
   result.success = false;
@@ -93,7 +93,7 @@ void StoreImpl(const base::FilePath& cache_dir,
                const std::string& key,
                const std::string& contents,
                base::TimeDelta age) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   MaybeCreateCache(cache_dir);
   if (contents.size() > kMaxPpdSizeBytes) {
     LOG(ERROR) << "Ignoring attempt to cache large object";

chromeos/printing/ppd_provider.cc

@@ -27,8 +27,8 @@
 #include "base/synchronization/lock.h"
 #include "base/task/post_task.h"
 #include "base/task_runner_util.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "base/threading/sequenced_task_runner_handle.h"
-#include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "chromeos/printing/ppd_cache.h"
@@ -251,7 +251,7 @@ bool PpdReferenceIsWellFormed(const Printer::PpdReference& reference) {
 bool FetchFile(const GURL& url, std::string* file_contents) {
   CHECK(url.is_valid());
   CHECK(url.SchemeIs("file"));
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   // Here we are un-escaping the file path represented by the url. If we don't
   // transform the url into a valid file path then the file may fail to be