post_build_ninja_summary.py detects a new incremental build by looking
for the end time stamps to go backwards. This means that repeated builds
that have a single long step will not be reliably detected and will, in
some cases, be missed entirely. This hits me sometimes when doing link
testing - delete an output DLL, rebuild, and the old results may be
displayed again.

This change updates the script to check for a different duration for an
existing record as well as an earlier end time should handle almost all
cases.

This also renames the targets local variable to avoid confusion due
to the targets member variable of the Targets class.

Bug: chromium:787983
Change-Id: I60a371df75d6cb7a55bd46b38156cb109feb8f15
Reviewed-on: https://chromium-review.googlesource.com/1061413
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>