[telemetry] Add support for BenchmarkResults that vary from page to page

tools/telemetry/telemetry/page_runner.py

Index: tools/telemetry/telemetry/page_runner.py
diff --git a/tools/telemetry/telemetry/page_runner.py b/tools/telemetry/telemetry/page_runner.py
index 09508b7f06020f88d5d3fa1793e2c63b5686dc8a..ea2b984d85427d38fcb416ae4b1a3e49dcf844d3 100644
--- a/tools/telemetry/telemetry/page_runner.py
+++ b/tools/telemetry/telemetry/page_runner.py
@@ -95,8 +95,10 @@ http://goto/read-src-internal, or create a new archive using record_wpr.
 
     # Check tracing directory.
     if options.trace_dir:
+      if not os.path.exists(options.trace_dir):
+        os.mkdir(options.trace_dir)
       if not os.path.isdir(options.trace_dir):
-        raise Exception('Trace directory doesn\'t exist: %s' %
+        raise Exception('--trace-dir isn\'t a directory: %s' %
                         options.trace_dir)
       elif os.listdir(options.trace_dir):
         raise Exception('Trace directory isn\'t empty: %s' % options.trace_dir)