ServiceWorker: Stop exposing ServiceWorkerContextCore

External components (eg. Push) can directly access SWContextCore, but the
context core is designed for internal use and should be accessed via
SWContextWrapper because it can be null in some cases (eg, failing to
restart the system. See [1,2] for details of the wrapper-core layering).

To enforce the rule, this CL stops exposing the context core and makes the
wrapper to provide interfaces for that instead.

[1] https://code.google.com/p/chromium/issues/detail?id=371675#c18
[2] https://docs.google.com/document/d/1eXdgnAOZC4dDDybmRpXT0t0lOYGhFf7xajOSanEb91Y/edit?usp=sharing

BUG=472019
TEST=compile

Review URL: https://codereview.chromium.org/1079923002

Cr-Commit-Position: refs/heads/master@{#326464}
14 files changed