[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chrome/browser/extensions/api/storage.

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=rdevlin.cronin@chromium.org

Bug: 874080
Change-Id: I4de4dd7b51419500492ecce093c5e82cd9fe6b35
Reviewed-on: https://chromium-review.googlesource.com/1191809Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593637}

chrome/browser/extensions/api/storage/managed_value_store_cache.cc

@@ -187,7 +187,6 @@ bool ManagedValueStoreCache::ExtensionTracker::UsesManagedStorage(
 void ManagedValueStoreCache::ExtensionTracker::LoadSchemasOnFileTaskRunner(
     std::unique_ptr<ExtensionSet> extensions,
     base::WeakPtr<ExtensionTracker> self) {
-  base::AssertBlockingAllowed();
   std::unique_ptr<policy::ComponentMap> components(new policy::ComponentMap);
 
   for (ExtensionSet::const_iterator it = extensions->begin();