v8: add setVariableValue debugger command

Issue 2399 part 1: In debugger allow modifying local variable values
Issue 2399 part 2: In debugger allow modifying local variable values

Review URL: https://codereview.chromium.org/11415042
Review URL: https://codereview.chromium.org/11412310

This is a back-port of upstream svn commits r13122 and r13202.