The mtime values I was seeing on my file system were sometimes
floating point numbers that were very very close but not exactly
the same. This might be due to numerical errors. Increasing the
tolerance to something still very small, precision is less than
a second. It will lead to fewer rebuilds of non-dirty files.

Bug: angleproject:2455
Change-Id: I95dc3214ee91af7a70a20cc625405e0e2bc18698
Reviewed-on: https://chromium-review.googlesource.com/1032855Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>