CacheStorage: Set the Response mime type based on stored content-header.

Previously the Response mime type was never set when reading from a
Cache.  This resulted in empty string blob.type values and could also
prevent code cache from being generated if a script Response was put
back into a Cache.

This CL makes us at least generate a mime type from the current
content-type header.  This is a partial solution since the mime type
should technically be frozen at initial Response construction time
even if the content-type header is later changed.  To support that we
will need to explicitly store a mime type on disk in the cache.  This
will be done later in crbug.com/938939.

Bug: 937963,938939
Change-Id: Iabfe3909ff35957501ef33132af2a0c1f70c5c94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506213
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638581}
5 files changed