Split cache: Refactoring cache key generation

This CL does not change any functionality.
In preparation for cache key to be computed in a higher layer
and passed to the URLRequest, this CL does the following refactoring:
1. Adds a member for cache key in URLRequest and HttpRequestInfo.
This is currently not getting filled anywhere.
2. The function to compute the cache key is refactored out to
HttpUtils so that it can be invoked in a URLRequest's creation layer.

Bug: 910708
Change-Id: I935f4f7f8b8d9822cb47681683b56bbb8b220ee9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605221
Commit-Queue: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659533}