share platform view slicing logic across iOS and Android. (#54010)

This removes support for "unobstructed platform views" on iOS - instead prefering to use the Android strategy of minimizing overlay layers, as this is generally more performant.
https://dart.googlesource.com/external/github.com/flutter/engine/+/3bba8a48ca140302942f8c918f6a5a6000df4808

DEPS

diff --git a/DEPS b/DEPS
index 1aa5980..77f3f00 100644
--- a/DEPS
+++ b/DEPS
@@ -280,7 +280,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + '91c259d5ee56c24fb870cf7be7d041bb9b8e139f',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '86f4938f419b498b004f4ad81e81e2af8c343638',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '3bba8a48ca140302942f8c918f6a5a6000df4808',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '4806f5cb0a2754a321947eab1be46494d099ea0f',
 

commits.json

diff --git a/commits.json b/commits.json
index ee423b2..325e17e 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"86f4938f419b498b004f4ad81e81e2af8c343638",
+  "engine/src/flutter":"3bba8a48ca140302942f8c918f6a5a6000df4808",
   "engine/src/flutter/third_party/dart":"4806f5cb0a2754a321947eab1be46494d099ea0f",
   "flutter":"f64190aacb1304ac1a4ba9c83f775d66d47b27c8"
 }
\ No newline at end of file