Compile fix after Add specialized content key class for resources

gcc (Ubuntu/Linaro 4.6.4-1ubuntu1~12.04) 4.6.4
error: comparison between signed and unsigned integer expressions

Review URL: https://codereview.chromium.org/871013002

src/gpu/GrResourceCache2.cpp

@@ -33,7 +33,7 @@ GrContentKey::Domain GrContentKey::GenerateDomain() {
     static int32_t gDomain = INHERITED::kInvalidDomain + 1;
 
     int32_t domain = sk_atomic_inc(&gDomain);
-    if (kInvalidDomain == gDomain) {
+    if (domain > SK_MaxU16) {
         SkFAIL("Too many Content Key Domains");
     }