Pass main thread WebThread to Platform::Initialize().

The main thread's WebThread is currently owned by each embedder, but we
plan to have it owned by Platform and eventually stop exposing WebThread
to non-Blink directories.

Currently, Platform::CurrentThread() is used for two purposes: (1) to
obtain the pointer to the main thread WebThread object, and (2) to
return the current thread's WebThread object in thread-local storage.
This patch gets rid of (1) and move it to Platform::Initialize()'s
argument, so we can easily shift the ownership of WebThread in the
future.

TBR=eroman@chromium.org,kinuko@chromium.org,dalecurtis@chromium.org,jcivelli@chromium.org

Bug: 826203
Change-Id: I93608b13f6b84a47112865da486647afc46ab039
Reviewed-on: https://chromium-review.googlesource.com/1154854
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579733}
14 files changed