Add WebFramesManager class.

The WebFrameManager class maintains a list of (and provides access to) WebFrame objects representing the currently rendered frames of the associated WebState.

Bug: 851636
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I62473ded9af401bf3b556e0d03776f9b67780d87
Reviewed-on: https://chromium-review.googlesource.com/1119661
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573639}
7 files changed