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Publishing the fuchsia.web.FrameHost introduces the possibility that
it causes the main web.Context to be created before the list of CORS-
exempt headers has been fetched. This can prevent headers being applied
based on UrlRequestRewriteRules, breaking later users of that Context.

StartComponent() is split into the public API which verifies arguments
and triggers fetch of the CORS exempt headers list, if necessary, and
a separate internal part called when the headers are available, which
actually creates the FrameHostComponent, or PendingCastComponent.

Bug: 1150488
Bug: b/173386485
Change-Id: I2f294e9f0acbf43c971004882461c0f38a4c37b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547580
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828844}