MemoryCache: Enable MemoryCache to have multiple isolated resource maps

This CL enables MemoryCache to store multiple resources for 1 URL so that
Service Worker can utilize the cache system.

Patch dependency
[1] Chromium(Get ServiceWorkerID): https://codereview.chromium.org/669303003/
[2] Blink(Get ServiceWorkerID): https://codereview.chromium.org/650033007/
[3] Blink(Tweak MemoryCache): THIS PATCH


BUG=423219
TEST=webkit_unit_tests --gtest_filter=MemoryCacheTest.*:ResourceFetcherTest.*:CachingCorrectnessTest.*:ImageResourceTest.*:RawResourceTest.*
TEST=run_webkit_tests.py --debug http/tests/serviceworker/chromium/memory-cache.html

Review URL: https://codereview.chromium.org/640463003

git-svn-id: svn://svn.chromium.org/blink/trunk@184613 bbb929c8-8fbe-4397-9dbb-9b2b20218538