Adding a probe fallback to HTTP test

tools/chrome_proxy/webdriver/common.py

Index: tools/chrome_proxy/webdriver/common.py
diff --git a/tools/chrome_proxy/webdriver/common.py b/tools/chrome_proxy/webdriver/common.py
index 2572a64a9253049576e6d5b6364a950ce18c46c3..47917bf87b868e6dfcf4c339ac97c63129a6d8c6 100644
--- a/tools/chrome_proxy/webdriver/common.py
+++ b/tools/chrome_proxy/webdriver/common.py
@@ -327,9 +327,18 @@ class TestDriver:
     """
     return self.ExecuteJavascript("return " + script, timeout)
 
-  def GetHistogram(self, histogram):
+  def GetHistogram(self, histogram, timeout=30):
+    """Gets a Chrome histogram as a dictionary object.
+
+    Args:
+      histogram: the name of the histogram to fetch
+      timeout: timeout for the underlying Javascript query.
+
+    Returns:
+      A dictionary object containing information about the histogram.
+    """
     js_query = 'statsCollectionController.getBrowserHistogram("%s")' % histogram
-    string_response = self.ExecuteJavascriptStatement(js_query)
+    string_response = self.ExecuteJavascriptStatement(js_query, timeout)
     self._logger.debug('Got %s histogram=%s', histogram, string_response)
     return json.loads(string_response)
 
@@ -382,6 +391,27 @@ class TestDriver:
     self._has_logs = False
     return all_messages
 
+  def SleepUntilHistogramHasEntry(self, histogram_name, sleep_intervals=10):
+    """Polls if a histogram exists in 1-6 second intervals for 10 intervals.
+    Allows script to run with a timeout of 5 seconds, so the default behavior
+    allows up to 60 seconds until timeout.
+
+    Args:
+      histogram_name: The name of the histogram to wait for
+      sleep_intervals: The number of polling intervals, each polling cycle takes
+      no more than 6 seconds.
+    Returns:
+      Whether the histogram exists
+    """
+    histogram = {}
+    while(not histogram and sleep_intervals > 0):
+      histogram = self.GetHistogram(histogram_name, 5)
+      if (not histogram):
+        time.sleep(1)
+        sleep_intervals -= 1
+
+    return bool(histogram)
+
   def GetHTTPResponses(self, include_favicon=False, skip_domainless_pages=True):
     """Parses the Performance Logs and returns a list of HTTPResponse objects.