Bug 1434230 part 4 - Enable Spectre string mitigations by default. r=luke

js/src/jit/JitOptions.cpp

@@ -233,7 +233,7 @@ DefaultJitOptions::DefaultJitOptions()
     }
 
     SET_DEFAULT(spectreIndexMasking, true);
-    SET_DEFAULT(spectreStringMitigations, false);
+    SET_DEFAULT(spectreStringMitigations, true);
 
     // Toggles whether unboxed plain objects can be created by the VM.
     SET_DEFAULT(disableUnboxedObjects, false);

modules/libpref/init/all.js

@@ -1552,7 +1552,7 @@ pref("javascript.options.dump_stack_on_debuggee_would_run", false);
 
 // Spectre security vulnerability mitigations.
 pref("javascript.options.spectre.index_masking", true);
-pref("javascript.options.spectre.string_mitigations", false);
+pref("javascript.options.spectre.string_mitigations", true);
 
 // Streams API
 pref("javascript.options.streams", false);