CacheStorage: Hold DOM binding objects alive during operations.

We are seeing some DCHECKs trigger that suggest the owning Cache object
is destroyed while an outstanding operation is in progress.  This can
in theory happen in code that operates on the cache, but then discards
its reference while waiting for the promise to settle.

Bug: 912141
Change-Id: Ic82e8b86bb427808df4d7f16326b50998cf6b031
Reviewed-on: https://chromium-review.googlesource.com/c/1364130
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614251}
2 files changed