ChildProcessLauncher without BrowserThread

This removes ChildProcessLauncher's nominal dependency on BrowserThread
initialization by capturing the client's current SequencedTaskRunner
rather than the current thread's BrowserThread::ID, which may not be
initialized yet.

Bug: 989769
Change-Id: Ia4b9cb0781540f6737be761c201b6440f7713ac6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731473
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#683390}
9 files changed