Don't treat this as a trivial expression.

This caused some regressions in Chromium.

BUG=http://crbug.com/36604
Review URL: http://codereview.chromium.org/652228

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3939 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ast.h

@@ -931,10 +931,6 @@ class VariableProxy: public Expression {
     return var()->is_global() || var()->rewrite()->IsLeaf();
   }
 
-  // Reading from a mutable variable is a side effect, but 'this' is
-  // immutable.
-  virtual bool IsTrivial() { return is_this(); }
-
   bool IsVariable(Handle<String> n) {
     return !is_this() && name().is_identical_to(n);
   }