ash: enforce that all Widgets have a context/parent when created

This will be important for SingleProcessMash as the context/parent is
needed to properly create the NativeWidgetDelegate. In particular
NativeWidgetAura's constructor takes an Env that comes from either the
context or parent. We need to ensure code in Ash sets that properly so that
the right Env is used.

BUG=847992
TEST=covered by tests

Change-Id: I2876ac862e00c4f03a7c328230db372f5d74a005
Reviewed-on: https://chromium-review.googlesource.com/1171204
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582322}
41 files changed