[TaskScheduler]: Migrate off of AssertBlockingAllowedDeprecated in /third_party/leveldatabase

base::AssertBlockingAllowedDeprecated is deprecated in favor of
ScopedBlockingCall, which serves as a precise annotation of
the scope that may/will block.

Please 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: The call might block (e.g. file I/O that might hit in memory cache).
      WILL_BLOCK: The call will definitely block (e.g. cache already checked and now pinging
        server synchronously).
    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().
  - Calls to blocking //base APIs don't need to be annotated
    with ScopedBlockingCall. All blocking //base APIs (e.g. base::ReadFileToString, base::File::Read,
    base::SysInfo::AmountOfFreeDiskSpace, base::WaitableEvent::Wait, etc.) have their
    own internal annotations.

Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1338391

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=pwnall@chromium.org

Bug: 903957
Change-Id: I81021e9c637e588b0cdcc464218df1770cedc635
Reviewed-on: https://chromium-review.googlesource.com/c/1338128Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608812}

third_party/leveldatabase/env_chromium.cc

@@ -29,7 +29,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/system/sys_info.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "base/trace_event/process_memory_dump.h"
@@ -73,7 +73,7 @@ static const char kDatabaseNameSuffixForRebuildDB[] = "__tmp_for_rebuild";
 static base::File::Error GetDirectoryEntries(const FilePath& dir_param,
                                              std::vector<FilePath>* result) {
   TRACE_EVENT0("leveldb", "ChromiumEnv::GetDirectoryEntries");
-  base::AssertBlockingAllowedDeprecated();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   result->clear();
 #if defined(OS_WIN)
   FilePath dir_filepath = dir_param.Append(FILE_PATH_LITERAL("*"));