Make multiple web bundle links behavior deterministic

Currently the behavior of multiple web bundle links is not
deterministic. It is because ResourceFetcher is keeping
SubresourceWebBundles in a HeapHashSet.

To fix this issue this CL changes the logc to use Vector in a new class
SubresourceWebBundleList, and use the last added link.

Bug: 1082020
Change-Id: I7cb0908f6b405c1d4982a6dc551d9bf7fc5fd201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2891272


Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882430}