Simplify initialization of utility thread.

Utility thread is a thread for utilities that make use of Blink.
Currently, the WebThread object for that thread is defined as
WebThreadImplForUtilityThread, but we can remove it if we initialize
Blink with blink::CreateMainThreadAndInitialize(), because the
utility thread does not require multi-thread nor other complex
functionalities.

UtilityBlinkPlatformImpl inherits from BlinkPlatformImpl, which is
a base class for blink::Platform defined in content/. This patch
makes it directly inherit from blink::Platform, since the utility
thread does not require functions defined in BlinkPlatformImpl --
BlinkPlatformImpl's functions are basically for full-fledged browser,
and we don't need them in the utility thread.

Bug: 826203
Change-Id: I57af86931af176ef5d1bd08d0f0ceb5e60d9d65b
Reviewed-on: https://chromium-review.googlesource.com/1175746
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584030}
13 files changed