[debugger] Remove dependency cycle

DebugCanvas.h and DebugLayerManger.h both included each other.
DebugCanvas.h already had a forward declaration of DebugLayerManager,
so that seemed the easiest way to break the cycle.

Change-Id: Ie8ce5eeca7aba2cdbbcbf92f9fdc42eccc319a2e
Bug: skia:12541
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/505641
Reviewed-by: Nathaniel Nifong <nifong@google.com>
2 files changed