[ServiceWorker] Change ServiceWorkerStatusCode::kMax to kMaxValue

This CL renames ServiceWorkerStatusCode::kMax to kMaxValue and sets its
value as the highest enumerator value, this avoids switch statements
having to handle a sentinel no-op value and makes it easier to use
UMA_HISTOGRAM_ENUMERATION().
Then, this CL revises all related tests.

BUG=789854

Change-Id: Ic0e9ad8f9bf90bb3e2b08dc424957a2f3a545c52
Reviewed-on: https://chromium-review.googlesource.com/1125532
Commit-Queue: Han Leon <leon.han@intel.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573664}
23 files changed