fix: re-entracy problem with InvalidateFrameSinkId().

[miniak]

Description of Change

Backport fix for https://bugs.chromium.org/p/chromium/issues/detail?id=907211
https://chromium-review.googlesource.com/c/chromium/src/+/1352525

Fix re-entracy problem with InvalidateFrameSinkId().

HostFrameSinkManager::InvalidateFrameSinkId() can make a synchronous IPC
to ensure whatever is drawing the platform window is destroyed before
the platform window gets destroyed. However, while waiting for the
synchronous IPC response other synchronous IPCs continue to get
processed. |frame_sink_data_map_| can get mutated in response to a
different synchronous IPC and |data| may no longer point to a valid
memory location when DestroyCompositorFrameSink() returns.

Change the order so that all modifications to |data| happen before the
synchronous IPC. Also add a comment to clarify that FrameSinkIds
shouldn't be reused after they are invalidated. We don't do this today
and it would cause more re-entrancy problems.

Also switch |frame_sink_data_map_| to use std::unordered_map instead of
base::flat_map. This map can get large (tens of elements per open tab)
so std::unordered_map is probably a better choice.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed re-entracy problem with InvalidateFrameSinkId().

/cc @electron/wg-security

[codebytere]

@miniak can you please fix conflicts so we can get this merged?

[alexeykuzmin]

@codebytere done

[release-clerk]

Release Notes Persisted

Fixed re-entracy problem with InvalidateFrameSinkId().