Allow #include of base/single_thread_task_runner.h in Blink.

The use of base::SingleThreadTaskRunner is already allowed according
to audit-non-blink-usage.py, but #include of the header was not
allowed except in Source/platform/.

This patch allows the include in the other places of Source/.

base::SingleThreadTaskRunner will be used much more frequently in
Blink, after we finish the conversion of WebTaskRunner. This change
prepares for it.

Bug: 763980, 794845
Change-Id: Iab0ed63f23efdfcb05df0b219bbed98e53f36f9c
Reviewed-on: https://chromium-review.googlesource.com/888300Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532093}

third_party/WebKit/Source/DEPS

@@ -8,6 +8,7 @@ include_rules = [
     "+base/macros.h",
     "+base/memory/ptr_util.h",
     "+base/memory/weak_ptr.h",
+    "+base/single_thread_task_runner.h",
     "+base/time/time.h",
     "+build",
     "+services/service_manager/public/cpp/connector.h",

third_party/WebKit/Source/platform/DEPS

@@ -21,7 +21,6 @@ include_rules = [
     "+base/numerics/safe_conversions.h",
     "+base/rand_util.h",
     "+base/run_loop.h",
-    "+base/single_thread_task_runner.h",
     "+base/strings/string_util.h",
     "+base/strings/stringprintf.h",
     "+base/synchronization/waitable_event.h",