[web] Platform views reset the 'cursor' set by the glasspane of the app.

[ditman]

Steps to Reproduce

The current implementation of platform views in the web engine introduce a very aggressive reset CSS stylesheet while in DOM-rendering mode:

• src/engine/html/platform_view.dart

This causes that changes to the cursor made by the flutter app (through the glass-pane) get reset once the mouse enters a platform view in the app (it seems to revert to cursor: default).

The fix is to inherit the cursor from the outside, after resetting all properties, with cursor: inherit;.

This happens currently near the tip of master:

$ flutter doctor -v
[✓] Flutter (Channel master, 1.24.0-8.0.pre.356, on Linux, locale en_US.UTF-8)
    • Flutter version 1.24.0-8.0.pre.356 at /usr/local/google/home/dit/github/flutter
    • Framework revision e6bd95bc5c (2 weeks ago), 2020-11-23 14:10:58 -0800
    • Engine revision 23a8e027db
    • Dart version 2.12.0 (build 2.12.0-62.0.dev)

[ditman]

This has rolled to flutter master. Forgot to add "Fixes" on the PR for this :)

[github-actions]

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