Add CastContentWindow Set*Context methods

This change introduces two SetContext() methods for CastContentWindow;
SetActivityContext() is used to supply common data for all activities,
while SetHostContext() provides data about the code that is hosting the
activity instance.

Merge-With: eureka-internal/239993

Bug: internal b/123538348
Test: CQ
Change-Id: I7cb4a313de78b0d511f4423dd5af4595a2866a7e
Reviewed-on: https://chromium-review.googlesource.com/c/1457722
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630499}
10 files changed