[build/android] Load/dump pickles directly form/to files

No need to build intermediate (possibly quite large) strings. This
might also help with large objects failing to pickle.

BUG=680935

Review-Url: https://codereview.chromium.org/2805533003
Cr-Original-Commit-Position: refs/heads/master@{#462553}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: be6dcf5eec3df0153f9e67b0b1a15cf4416144b7

android/pylib/instrumentation/instrumentation_test_instance.py

@@ -321,8 +321,8 @@ def _GetTestsFromPickle(pickle_path, jar_path):
     raise TestListPickleException(
         '%s newer than %s.' % (jar_path, pickle_path))
 
-  with open(pickle_path, 'r') as pickle_file:
-    pickle_data = pickle.loads(pickle_file.read())
+  with open(pickle_path, 'r') as f:
+    pickle_data = pickle.load(f)
   jar_md5 = md5sum.CalculateHostMd5Sums(jar_path)[jar_path]
 
   if pickle_data['VERSION'] != _PICKLE_FORMAT_VERSION:

android/pylib/local/device/local_device_perf_test_run.py

@@ -206,10 +206,10 @@ class TestShard(object):
     pickled = os.path.join(constants.PERF_OUTPUT_DIR, result['name'])
     if os.path.exists(pickled):
       with file(pickled, 'r') as f:
-        previous = pickle.loads(f.read())
+        previous = pickle.load(f)
         result['output'] = previous['output'] + result['output']
     with file(pickled, 'w') as f:
-      f.write(pickle.dumps(result))
+      pickle.dump(result, f)
 
   def _TestTearDown(self):
     if self._output_dir:

android/pylib/perf/perf_test_instance.py

@@ -26,7 +26,7 @@ def _GetPersistedResult(test_name):
     return None
 
   with file(file_name, 'r') as f:
-    return pickle.loads(f.read())
+    return pickle.load(f)
 
 
 def _GetChromiumRevision():