[Chromecast] Make MatrixID enum uint8_t-based to match VideoColorSpace

Review-Url: https://codereview.chromium.org/2807523003
Cr-Commit-Position: refs/heads/master@{#462877}

chromecast/public/media/decoder_config.h

@@ -212,7 +212,7 @@ enum class TransferID : uint8_t {
 };
 
 // Table 4
-enum class MatrixID : int8_t {
+enum class MatrixID : uint8_t {
   RGB = 0,
   BT709 = 1,
   UNSPECIFIED = 2,