check the range of exit codes returned from run-webkit-tests.

Normally if run-webkit-tests detects regressions, it returns the number
of regressions in the exit code (i.e., if 5 tests fail, the exit code is 5).
We also use a few magic values to indicate other errors (like 130 for
interrupted, -1/254 for internal error, etc.). This means that if we get
a lot of errors, we can overlap and produce confusing results.

This change adds a range check on the number of errors; if we get more than
101 failures, we limit the exit code to 101. This should prevent overlap
while preserving behavior in all but very abnormal circumstances.

Arguably a better change would just be to use 1 for the error and ignore
the number of failures altogether, but that might break scripts so I'm
not going to do this now.

TBR=scottmg@chromium.org
BUG=309197

Review URL: https://codereview.chromium.org/185503002

git-svn-id: svn://svn.chromium.org/blink/trunk@168251 bbb929c8-8fbe-4397-9dbb-9b2b20218538