task posting v3: moving away from SequencedTaskRunnerHandle

To continue the migration away from TaskRunnerHandles, the codebase
was refactored using the following scripts:
shell script:
https://paste.googleplex.com/4673967729147904
python:
https://paste.googleplex.com/5302682490241024

This will do a few sed-like modifications, changing calls to methods of
SequencedTaskRunnerHandle to calls to methods of
SequencedTaskRunner::CurrentDefaultHandle, and swapping includes.

Bug: 1026641
AX-Relnotes: n/a.
Change-Id: I49e50a2bd1e78b00e7c067219fff96d2e0bc0b46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3983373
Commit-Queue: Gabriel Charette <gab@chromium.org>
Owners-Override: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071032}

components/safe_search_api/fake_url_checker_client.cc

@@ -7,7 +7,7 @@
 #include <utility>
 
 #include "base/callback.h"
-#include "base/threading/sequenced_task_runner_handle.h"
+#include "base/task/sequenced_task_runner.h"
 
 namespace safe_search_api {
 
@@ -28,7 +28,7 @@ void FakeURLCheckerClient::RunCallback(ClientClassification classification) {
 
 void FakeURLCheckerClient::RunCallbackAsync(
     ClientClassification classification) {
-  base::SequencedTaskRunnerHandle::Get()->PostTask(
+  base::SequencedTaskRunner::GetCurrentDefault()->PostTask(
       FROM_HERE, base::BindOnce(std::move(callback_), url_, classification));
 }