Add kEmptyImmutableString.

We can use this instead of ImmutableString("").

Bug: angleproject:2665
Change-Id: I8b3d5d3075838b9f2caa1627071202e48a5fdc83
Reviewed-on: https://chromium-review.googlesource.com/1108085
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
diff --git a/src/compiler/translator/Symbol.cpp b/src/compiler/translator/Symbol.cpp
index 12d0ef3..b2c4110 100644
--- a/src/compiler/translator/Symbol.cpp
+++ b/src/compiler/translator/Symbol.cpp
@@ -184,7 +184,7 @@
     mParametersVector->push_back(p);
     mParameters  = mParametersVector->data();
     mParamCount  = mParametersVector->size();
-    mMangledName = ImmutableString("");
+    mMangledName = kEmptyImmutableString;
 }
 
 void TFunction::shareParameters(const TFunction &parametersSource)