Make `benchmark.mojo` resilient to incorrect trace events.

This patch majes `benchmark.mojo` print a warning and ignore incorrect
events instead of failing hard.

The warnings are surfaced in `mojo_benchmark`.

Fixes #425.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1343413002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 447d2f9cbe19366c55e0d192e0378ca95c4bfc56

mojo_benchmark

diff --git a/mojo_benchmark b/mojo_benchmark
index a8191f5..5373d35 100755
--- a/mojo_benchmark
+++ b/mojo_benchmark
@@ -97,7 +97,7 @@
 
   # Echo measurement results.
   for line in output_lines:
-    if line.strip().startswith('measurement:'):
+    if line.strip().startswith('measurement:') or 'WARNING' in line:
       print line
   return True