Add check for trivial string equality

BUG=

Review-Url: https://codereview.chromium.org/2338623005
diff --git a/core/fxcrt/fx_basic_wstring.cpp b/core/fxcrt/fx_basic_wstring.cpp
index 8837c6b..88fc588 100644
--- a/core/fxcrt/fx_basic_wstring.cpp
+++ b/core/fxcrt/fx_basic_wstring.cpp
@@ -180,6 +180,9 @@
 }
 
 bool CFX_WideString::operator==(const CFX_WideString& other) const {
+  if (m_pData == other.m_pData)
+    return true;
+
   if (IsEmpty())
     return other.IsEmpty();