Regressed: https://chromium-review.googlesource.com/531798

This was because the std::stringstream constructor was being called
every draw call, for the InfoLog of the dynamically recompiled
shaders in the D3D back-ends. The constructor was allocating memory
and freeing it on destruction. Instead use a lazy init for the
std::stringstream in InfoLog like we do for Error.

BUG=chromium:750685

Change-Id: I9cd429ae4e1439ae504943a9cad31cbbed17ef32
Reviewed-on: https://chromium-review.googlesource.com/594629
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>