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This removes the string parameters from the interfaces.
It also removes the json dictionaries that were representing
  the state.
v8 session continues to be encoded as JSON
  (see inspector_session.{h,cc}).

For reference, the cl that introduced the new way to manage
the agent state was this one (and advantage is that the updates
sent back to the browser process are deltas, not the entire
state):
https://chromium-review.googlesource.com/c/chromium/src/+/1149201
This change can get rid of the old way since all agents have
been migrated to the new way.

Bug: 851762
Change-Id: Ie245c1c073eb006d4db19d819c2882e6a36f5b2b
Reviewed-on: https://chromium-review.googlesource.com/1162931Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581103}