Add stub of sparse_histogram.h Very similar to histogram_macros.h, but they're split in Chromium, so add this one too. R=mark@chromium.org BUG=crashpad:100 Change-Id: I92da6b6165410e8b6c602be3474133cc187777f1 Reviewed-on: https://chromium-review.googlesource.com/386310 Reviewed-by: Mark Mentovai <mark@chromium.org>