Bug 1600963 - reduce size of principal KeyVals; r=mccr8

By declaring the individual enum types to fit in a `uint8_t`, and
rearranging the templated `KeyValT` struct, we can pack `KeyValT`
to save a word of storage.

Differential Revision: https://phabricator.services.mozilla.com/D55687

--HG--
extra : moz-landing-system : lando

caps/BasePrincipal.h

@@ -261,9 +261,10 @@ class BasePrincipal : public nsJSPrincipals {
   template<typename SerializedKey>
   struct KeyValT
   {
+    static_assert(sizeof(SerializedKey) == 1, "SerializedKey should be a uint8_t");
+    SerializedKey key;
     bool valueWasSerialized;
     nsCString value;
-    SerializedKey key;
   };
 
  private:

caps/ContentPrincipal.h

@@ -59,7 +59,7 @@ class ContentPrincipal final : public BasePrincipal {
 
   virtual nsresult PopulateJSONObject(Json::Value& aObject) override;
   // Serializable keys are the valid enum fields the serialization supports
-  enum SerializableKeys { eURI = 0, eDomain, eSuffix, eMax = eSuffix };
+  enum SerializableKeys : uint8_t { eURI = 0, eDomain, eSuffix, eMax = eSuffix };
   typedef mozilla::BasePrincipal::KeyValT<SerializableKeys> KeyVal;
 
   static already_AddRefed<BasePrincipal> FromProperties(

caps/ExpandedPrincipal.h

@@ -62,7 +62,7 @@ class ExpandedPrincipal : public nsIExpandedPrincipal,
 
   virtual nsresult PopulateJSONObject(Json::Value& aObject) override;
   // Serializable keys are the valid enum fields the serialization supports
-  enum SerializableKeys { eSpecs = 0, eSuffix, eMax = eSuffix };
+  enum SerializableKeys : uint8_t { eSpecs = 0, eSuffix, eMax = eSuffix };
   typedef mozilla::BasePrincipal::KeyValT<SerializableKeys> KeyVal;
 
   static already_AddRefed<BasePrincipal> FromProperties(

caps/NullPrincipal.h

@@ -86,7 +86,7 @@ class NullPrincipal final : public BasePrincipal {
   virtual nsresult PopulateJSONObject(Json::Value& aObject) override;
 
   // Serializable keys are the valid enum fields the serialization supports
-  enum SerializableKeys { eSpec = 0, eSuffix, eMax = eSuffix };
+  enum SerializableKeys : uint8_t { eSpec = 0, eSuffix, eMax = eSuffix };
   typedef mozilla::BasePrincipal::KeyValT<SerializableKeys> KeyVal;
 
   static already_AddRefed<BasePrincipal> FromProperties(