Always write //build/util/LASTCHANGE.committime.

When we added LASTCHANGE.committime in r583420, we missed a
case where we might not write it in the hook, and in that
situation the build would fail due to the file being missing.

TBR=thakis@chromium.org
BUG=875039

Change-Id: I5c596f55fe90bc5f5175c74ba27e218f108f843b
Reviewed-on: https://chromium-review.googlesource.com/1178892Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583852}

build/util/lastchange.py

@@ -214,8 +214,10 @@ def main(argv=None):
       sys.stdout.write(contents)
     else:
       if out_file:
-        if WriteIfChanged(out_file, contents):
-          with open(out_file + '.committime', 'w') as timefile:
+        committime_file = out_file + '.committime'
+        out_changed = WriteIfChanged(out_file, contents)
+        if out_changed or not os.path.exists(committime_file):
+          with open(committime_file, 'w') as timefile:
             timefile.write(str(version_info.timestamp))
       if header:
         WriteIfChanged(header,