[fuchsia] Recreate web.Context if persisted cache is erased.

CastRunner's main web.Context is supplied with persistent web profile
and CDM data directories backed by isolated cache storage.  If the cache
is erased then the directories must therefore be re-created, and the
web.Context restarted.

Since there is no explicit notification when the cache is erased, a
sentinel file is created alongside the directories, and its existence
checked-for whenever a new component is to be launched.

Bug: 1188780
Bug: b/181713070
Change-Id: I9280c9b0abc6f4239c7832f1c754a3eb51820f19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2775937


Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864889}