Bug 1309109 part 1 - Change underlying type of StyleBackendType to uint8_t. r=heycam

MozReview-Commit-ID: B6TmOq6elfs

--HG--
extra : source : f7771920c448c382f215356275cb321a6943972e

layout/style/StyleBackendType.h

@@ -12,7 +12,7 @@ namespace mozilla {
 /**
  * Enumeration that represents one of the two supported style system backends.
  */
-enum class StyleBackendType : int
+enum class StyleBackendType : uint8_t
 {
   Gecko = 1,
   Servo