Roll Flutter Engine from 39ae00bf5328 to e6b872b01fde (2 revisions) (#139573)

https://github.com/flutter/engine/compare/39ae00bf5328...e6b872b01fde

2023-12-05 jonahwilliams@google.com [Impeller] Render emoji shadows with correct colors. (flutter/engine#48593)
2023-12-05 jonahwilliams@google.com [Impeller] Document what some of this shader metadata is. (flutter/engine#48647)

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 chinmaygarde@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

-39ae00bf5328c28a2fe8d448bccda2138a41833b
+e6b872b01fde134fa90d24d3e9a7c4e666fb9180