[generators] Add some explanation on forcing context allocation.

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index d7d8162205ac5f8edf8e3a0f9bd16dc67cb39b0d..d1dc4944b8d5c2c88e90237a42e7fffb231f4b0e 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -4103,7 +4103,8 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
     if (is_generator) {
       // For generators, allocating variables in contexts is currently a win
       // because it minimizes the work needed to suspend and resume an
-      // activation.
+      // activation.  The machine code produced for generators (by full-codegen)
+      // relies on this forced context allocation, but not in an essential way.

[Michael Starzinger, 2016/03/17 12:48:00]

nit: The "[...], but not in an essential way" part is misleading. Can we just
point to the place where it is relied upon by saying something along the lines
of "[...], because arguments on the stack to the generator are not restored when
resuming a generator" instead. WDYT?

       scope_->ForceContextAllocation();
 
       // Calling a generator returns a generator object.  That object is stored