When NINJA_SUMMARIZE_BUILD=1 then autoninja prints a summary of build
performance at the end. This change makes it so that ninja prints more
detailed build statistics during the build. In particular, slow process
creation is a common but difficult to see bottleneck. This change makes
it so that if NINJA_SUMMARIZE_BUILD=1 then the number of running build
processes will be displayed.

Bug: 787983
Change-Id: Ic0907e23e7f762e23e4795059b37085e0cb8c4ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1582802Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>