[Read Anything] Introduce the ReadAnythingAppController.

ReadAnythingAppController is a C++ class in the renderer process.
It serves two purposes:
1. Communicate with ReadAnythingPageHandler (C++) via mojom.
2. Communicate with ReadAnythingApp (ts) via gin bindings.
The ReadAnythingAppController unserializes the AXTreeSnapshot and
exposes methods to the ts resource (ReadAnythingApp) for accessing
information about the AXTree.

This CL is first in a series of CLs to unserialize the AXTreeUpdate
in the renderer process in C++ and use gin bindings to enable the ts
resource to query the AXTree. The reasoning for this is 3-fold:
1. The AXTree::Unserialize method is complex (it performs tree
   manipulation) and is written in C++.
2. The AXTree::Unserialize method is a performance bottleneck and so
   when possible it is best to avoid using it in the main (browser)
   process.
3. The feature will require manipulation of some values in the AXTree
   which must be done in an unprivileged process.

More information here: https://docs.google.com/document/d/1CV2IX56z5FuUCiuNsBcnKyZ5_0Zd0_LZ4dBora93utc/edit#heading=h.30lobn7jy68d

Bug: 1266555
Change-Id: I15cac25e2955817d1a1c79e81e186d6f4fed9322
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3710123
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Abigail Klein <abigailbklein@google.com>
Cr-Commit-Position: refs/heads/main@{#1016884}