Set NINJA_STATUS for better build performance summarizing

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>

autoninja.bat

@@ -3,9 +3,17 @@
 :: Use of this source code is governed by a BSD-style license that can be
 :: found in the LICENSE file.
 
+setlocal
+
 REM Set unique build ID.
 FOR /f "usebackq tokens=*" %%a in (`python -c "import uuid; print uuid.uuid4()"`) do set AUTONINJA_BUILD_ID=%%a
 
+REM If a build performance summary has been requested then also set NINJA_STATUS
+REM to trigger more verbose status updates. In particular this makes it possible
+REM to see how quickly process creation is happening - often a critical clue on
+REM Windows. The trailing space is intentional.
+if "%NINJA_SUMMARIZE_BUILD%" == "1" set NINJA_STATUS=[%%r processes, %%f/%%t @ %%o/s : %%es ] 
+
 REM Execute whatever is printed by autoninja.py.
 REM Also print it to reassure that the right settings are being used.
 FOR /f "usebackq tokens=*" %%a in (`python %~dp0autoninja.py "%*"`) do echo %%a & %%a