sandwich: Fixes a bug when served from cache request are before.

Before, prefetch experiment's CSV collection was maintaining a dict
of transfered bytes per URLs as it was iterating over all resource
requests and served from memory cache URLs request would just grabe
the size from this dictionary. But we may have requests served from
memory cache before original request that filled the memory cache,
leading to a KeyError.

This CL fixes this issue by first building the dictionary by going
over resources requests, and then actually sum the bytes using this
dictionary, avoiding the KeyError.

BUG=582080

Review-Url: https://codereview.chromium.org/2134763002
Cr-Commit-Position: refs/heads/master@{#404626}

tools/android/loading/sandwich_prefetch.py

@@ -414,16 +414,14 @@ def _ProcessRunOutputDir(
     run_output_verifier.VerifyTrace(trace)
 
     logging.info('extracting metrics from trace: %s', trace_path)
-    served_from_network_bytes = 0
-    served_from_cache_bytes = 0
-    urls_hitting_network = set()
+
+    # Gather response size per URLs.
     response_sizes = {}
     for request in sandwich_utils.FilterOutDataAndIncompleteRequests(
         trace.request_track.GetEvents()):
       # Ignore requests served from the blink's cache.
       if request.served_from_cache:
         continue
-      urls_hitting_network.add(request.url)
       if request.from_disk_cache:
         if request.url in cached_encoded_data_lengths:
           response_size = cached_encoded_data_lengths[request.url]
@@ -433,13 +431,27 @@ def _ProcessRunOutputDir(
           # load.
           logging.warning('Looks like could be served from memory cache: %s',
               request.url)
-          response_size = response_sizes[request.url]
-        served_from_cache_bytes += response_size
+          if request.url in response_sizes:
+            response_size = response_sizes[request.url]
       else:
         response_size = request.GetResponseTransportLength()
-        served_from_network_bytes += response_size
       response_sizes[request.url] = response_size
 
+    # Sums the served from cache/network bytes.
+    served_from_network_bytes = 0
+    served_from_cache_bytes = 0
+    urls_hitting_network = set()
+    for request in sandwich_utils.FilterOutDataAndIncompleteRequests(
+        trace.request_track.GetEvents()):
+      # Ignore requests served from the blink's cache.
+      if request.served_from_cache:
+        continue
+      urls_hitting_network.add(request.url)
+      if request.from_disk_cache:
+        served_from_cache_bytes += response_sizes[request.url]
+      else:
+        served_from_network_bytes += response_sizes[request.url]
+
     # Make sure the served from blink's cache requests have at least one
     # corresponding request that was not served from the blink's cache.
     for request in sandwich_utils.FilterOutDataAndIncompleteRequests(