[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>

tools/debugger/BUILD.bazel

@@ -5,7 +5,6 @@ generated_cc_atom(
     hdrs = ["DebugCanvas.h"],
     visibility = ["//:__subpackages__"],
     deps = [
-        ":DebugLayerManager_hdr",
         ":DrawCommand_hdr",
         "//include/core:SkCanvasVirtualEnforcer_hdr",
         "//include/core:SkCanvas_hdr",

tools/debugger/DebugCanvas.h

@@ -16,7 +16,6 @@
 #include "include/pathops/SkPathOps.h"
 #include "include/private/SkTArray.h"
 #include "tools/UrlDataManager.h"
-#include "tools/debugger/DebugLayerManager.h"
 #include "tools/debugger/DrawCommand.h"
 
 #include <map>