Take care fall-through case of switch in skia

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1549273002

Review URL: https://codereview.chromium.org/1549273002

include/gpu/GrTestUtils.h

@@ -91,6 +91,7 @@ static inline uint8_t GrRandomCoverage(SkRandom* random) {
     switch (colorMode) {
         case kZero_CoverageMode:
             coverage = 0;
+            break;
         case kAllOnes_CoverageMode:
             coverage = 0xff;
             break;

include/gpu/GrTypesPriv.h

@@ -178,6 +178,7 @@ static inline GrSLType GrVertexAttribTypeToSLType(GrVertexAttribType type) {
     switch (type) {
         default:
             SkFAIL("Unsupported type conversion");
+            return kVoid_GrSLType;
         case kUByte_GrVertexAttribType:
         case kFloat_GrVertexAttribType:
             return kFloat_GrSLType;