Reland "[web] Render in custom target (#37738)"

[ditman]

This PR fixes an html renderer bug introduced in the feature above that was breaking the following flutter/flutter golden tests:

*  24 - value_indicating_slider_test.dart (material.slider_...)
*   1 - material/magnifier_test.dart (material.magnifier.position...)
*   8 - snack_bar_test.dart (material.snack_bar...)
*   1 - material/text_field_test.dart (material.text_field_golden...) (missing 'debug' label?)
*   2 - viewport_test.dart (rendering.shrink_wrap...)
*   2 - star_border_test.dart (painting.star_border....)
*   2 - image_filter_test.dart (widgets.image_filter_blur_...) (antialias? _offset?)
*   1 - page_route_builder_test.dart (widgets.page_route_builder...)
*   1 - overscroll_stretch_indicator_test.dart (widgets.overscroll_...)
*   1 - physical_model_test.dart (widgets.physical_model_overflow...) (antialias?)
*   1 - list_wheel_scroll_view_test.dart (widgets.list_wheel_scroll_view...)
*   1 - cupertino/magnifier_test.dart (cupertino_magnifier.position...)

This PR relands #37738 (by reverting #38469, and adding a small fix).

Find attached the resulting "golden" files used to verify that the above tests would now pass:

• local_golden_results.zip

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[yjbanov]