Make net::SharedDictionary RefCounted

We are planning to introduce a new preload dictionary feature. We are
also planning to introduce logic to avoid using a dictionary if it is
preloaded but the binary has not yet been preloaded.

To implement this feature, we need to be able to check the equality of
the preloaded dictionary and the dictionary for the network request.

Therefore, this CL changes net::SharedDictionary to RefConuted. We can
check the equality of the dictionary by just checking the pointer of
SharedDictionary.

Bug: 349030549
Change-Id: I813ddbc29bb6f346891f664957d15a5fdd0b521e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5700565
Reviewed-by: Patrick Meenan <pmeenan@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1327954}