appcache: Move AppCacheFrontend from content/common to content/browser.

AppCacheFrontend is only used in AppCache's browser-side
implementation. Having it in content/common/appcache_interfaces.h is
misleading, as it suggests that the class is shared with the renderer.

Bug: 611938
Change-Id: I68865d63ee38841d59b61866664716c0ec8c2c39
Reviewed-on: https://chromium-review.googlesource.com/c/1429247
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625399}
15 files changed