Remove setModifiersHandle, restore const-ness to Symbols

Change-Id: I40ceb5cab0473c08c92fbb6aa9afad6173c0fb37
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325624
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed