[google_maps_flutter_web] Help web integration tests pass.

[ditman]

• Update mocks (they had drifted with the latest changes)
• Make Markers/Clustering target types less specific (remove extends JSObject) so mockito can mock them.

Removing the T extends JSObject from some of the code that we intend to mock with mockito is needed, because mockito can't generate JSObjects, and we get errors like this:

Launching integration_test/google_maps_controller_test.dart on Web Server in debug mode...
org-dartlang-app:/google_maps_controller_test.mocks.dart:282:16: Error: Type argument 'Object?' doesn't conform to
the bound 'JSObject' of the type variable 'T' on 'MarkersController'.
 - 'Object' is from 'dart:core'.
Try changing type arguments so that they conform to the bounds.
    implements _i2.MarkersController<Object?, Object> {
               ^
../lib/src/markers.dart:13:34: Context: This is the type variable whose bound isn't conformed to.
abstract class MarkersController<T extends JSObject, O>
                                 ^

/cc @aednlaxer

[aednlaxer]

Fantastic, thanks!