Use SkToInt to avoid warning in Flutter roll.

The Flutter roll was failing due to -Wsign-compare.

Bug: chromium:1505053
Change-Id: Id12876f6f97682466f19b56cfa562366380f27cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/783036
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
(cherry picked from commit 0eea0b277d7d35e4c2612646d7dfe507341e337e)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/786197
Reviewed-by: Heather Miller <hcm@google.com>
Owners-Override: Heather Miller <hcm@google.com>
Auto-Submit: Heather Miller <hcm@google.com>

src/gpu/ganesh/ops/DrawMeshOp.cpp

diff --git a/src/gpu/ganesh/ops/DrawMeshOp.cpp b/src/gpu/ganesh/ops/DrawMeshOp.cpp
index 34dfd1d..7e875af 100644
--- a/src/gpu/ganesh/ops/DrawMeshOp.cpp
+++ b/src/gpu/ganesh/ops/DrawMeshOp.cpp
@@ -1037,7 +1037,7 @@
     if (SkToBool(fIndexCount) != SkToBool(that->fIndexCount)) {
         return CombineResult::kCannotCombine;
     }
-    if (SkToBool(fIndexCount) && fVertexCount > UINT16_MAX - that->fVertexCount) {
+    if (SkToBool(fIndexCount) && fVertexCount > SkToInt(UINT16_MAX) - that->fVertexCount) {
         return CombineResult::kCannotCombine;
     }