Bug 1502053 - Parse default prefs from MOZ_DEFAULT_PREFS env var r=njn

This will be used by GeckoView to set initial pref values that would
normally come from prefs.js or similar on desktop and Fennec.

Differential Revision: https://phabricator.services.mozilla.com/D9820

modules/libpref/Preferences.cpp

@@ -3231,6 +3231,9 @@ Preferences::HandleDirty()
 static nsresult
 openPrefFile(nsIFile* aFile, PrefValueKind aKind);
 
+static nsresult
+parsePrefData(const nsCString& aData, PrefValueKind aKind);
+
 // clang-format off
 static const char kPrefFileHeader[] =
   "// Mozilla User Preferences"
@@ -3890,6 +3893,11 @@ Preferences::GetInstanceForService()
     }
   }
 
+  const char* defaultPrefs = getenv("MOZ_DEFAULT_PREFS");
+  if (defaultPrefs) {
+    parsePrefData(nsCString(defaultPrefs), PrefValueKind::Default);
+  }
+
   gCacheDataDesc = "set by GetInstanceForService() (2)";
 
   // Preferences::GetInstanceForService() can be called from GetService(), and
@@ -4619,6 +4627,20 @@ openPrefFile(nsIFile* aFile, PrefValueKind aKind)
   return NS_OK;
 }
 
+static nsresult
+parsePrefData(const nsCString& aData, PrefValueKind aKind)
+{
+  TimeStamp startTime = TimeStamp::Now();
+  const nsCString path = NS_LITERAL_CSTRING("$MOZ_DEFAULT_PREFS");
+
+  Parser parser;
+  if (!parser.Parse(path, aKind, path.get(), startTime, aData)) {
+    return NS_ERROR_FILE_CORRUPTED;
+  }
+
+  return NS_OK;
+}
+
 static int
 pref_CompareFileNames(nsIFile* aFile1, nsIFile* aFile2, void* /* unused */)
 {