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More specifically, this CL:
* Adds a mojom file to connect the web frontend part and c++ backend.
* Adds c++ classes implementing web ui controller and some mojom interfaces.
* Adds html and js files implementing the skeleton of the web page, including
  BrowserProxy, which connects the Renderer JS to the backend C++.
* Adds code to display the web page at chrome://crostini-installer if feature
  CrostiniWebUIInstaller is enabled
* Adds skeleton for browser test.

Bug: 929571
Change-Id: Ie9b0931862ec5e40bc21ea7683ffc873e4034f1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778884Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Auto-Submit: Jason Lin <lxj@google.com>
Commit-Queue: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#695037}