Restore variadic macros in DevToolsEmbedderMessageDispatcher
The variadic macros in DevToolsEmbedderMessageDispatcher were removed
because pre-RTM VS 2015 couldn't handle them. RTM VS 2015 can, so I'm
putting them back.
I've verified that the two changes added to this file since the removal
are still there and that they are the only changes relative to the state
of the file before the removal. I've verified that the code compiles
with VS 2015.
R=tsepez@chromium.org
BUG=491973
Review URL: https://codereview.chromium.org/
1354953003
Cr-Commit-Position: refs/heads/master@{#349793}