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facilitates WebUI creation.

All parts of WUG are implemented:
* Base classes for view (View) and view-model (ViewModel)
* WebUIView -- specialization of View for use in WebUI.
* View's CC, HTML and JS code generator
* Templates for GYP and GN which help to generate code and
  produce components from generated code.

Tests to follow.

TEST=none
TBR=avi@chromium.org,mark@chromium.org
BUG=459230

Review URL: https://codereview.chromium.org/928163002

Cr-Commit-Position: refs/heads/master@{#318884}