ServiceWorker: Migrate the script cache backend from BlockFile to Simple
This CL switches the backend of ServiceWorkerDiskCache from BlockFile to Simple,
and migrates cached resources before initializing the diskcache.
* DiskCache migration consists of 2 parts:
1) Copying contents into a new diskcache. This should be complete before
starting diskcache initialization.
2) Lazily removing an old diskcache after migration is complete.
* These migration states are recorded in ServiceWorkerDatabase. If migration
is aborted in the middle of the execution, the retry will happen on the next
initialization.
* If migration fails, DeleteAndStartOver runs and creates a new storage.
BUG=487482
TEST=content_unittests --gtest_filter=ServiceWorker*
TEST=content_browsertests --gtest_filter=ServiceWorker*
TEST=manual (run Chromium with an old diskcache and make sure migration correctly happens)
Review URL: https://codereview.chromium.org/1152543002
Cr-Commit-Position: refs/heads/master@{#334811}
15 files changed
