Fix signed/unsigned mismatch in shaper assert.

Change-Id: I05e0e3f5940281b8c9fdcbabf6d8c1013bfa48d4
Reviewed-on: https://skia-review.googlesource.com/c/192824
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/modules/skshaper/src/SkShaper_harfbuzz.cpp b/modules/skshaper/src/SkShaper_harfbuzz.cpp
index 2d0a8d9..99553da 100644
--- a/modules/skshaper/src/SkShaper_harfbuzz.cpp
+++ b/modules/skshaper/src/SkShaper_harfbuzz.cpp
@@ -955,7 +955,7 @@
                 size_t modelStartCluster = utf8Start - utf8;
                 for (int i = 0; i < model.fNumGlyphs; ++i) {
                     SkASSERT(modelStartCluster <= model.fGlyphs[i].fCluster);
-                    SkASSERT(                     model.fGlyphs[i].fCluster < utf8End - utf8);
+                    SkASSERT(                     model.fGlyphs[i].fCluster < (size_t)(utf8End - utf8));
                     if (!model.fGlyphs[i].fUnsafeToBreak) {
                         modelText[model.fGlyphs[i].fCluster - modelStartCluster].glyphLen = i;
                         modelText[model.fGlyphs[i].fCluster - modelStartCluster].advance = advance;