Roll Flutter Engine from 1b842ae58b3d to a46a7b273a5b (2 revisions) (#145569)

https://github.com/flutter/engine/compare/1b842ae58b3d...a46a7b273a5b

2024-03-21 737941+loic-sharma@users.noreply.github.com [Fuchsia] Assert renders are only to the implicit view (flutter/engine#51559)
2024-03-21 737941+loic-sharma@users.noreply.github.com [Embedder API] Fix test helper's present callback registration (flutter/engine#51560)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC jonahwilliams@google.com,rmistry@google.com,zra@google.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Flutter: https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

bin/internal/engine.version

-1b842ae58b3d2de6f67fe55f19ae641f0330184e
+a46a7b273a5b796194bb3d8a5c1f4c7ab27e7365