Disable isolate_dependency_tracked in build driver for .isolate files.

build/isolate.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file is meant to be included into a target to provide a rule
 # to "build" .isolate files into a .isolated file.
 #
 # To use this, create a gyp target with the following form:
 # 'conditions': [
 #   ['test_isolation_mode != "noop"', {
@@ skipping 22 matching lines @@
 #
 # The generated .isolated file will be:
 #   <(PRODUCT_DIR)/foo_test.isolated
 
 {
   'rules': [
     {
       'rule_name': 'isolate',
       'extension': 'isolate',
       'inputs': [
-        '<@(isolate_dependency_tracked)',
         '<(RULE_INPUT_PATH)',
+        # Disable file tracking by the build driver for now. This means the
+        # project must have the proper build-time dependency for their runtime
+        # dependency. This improves the runtime of the build driver since it
+        # doesn't have to stat() all these files.
+        #
+        # More importantly, it means that even if a isolate_dependency_tracked
+        # file is missing, for example if a file was deleted and the .isolate
+        # file was not updated, that won't break the build, especially in the
+        # case where foo_tests_run is not built! This should be reenabled once
+        # the switch-over to running tests on Swarm is completed.
+        #'<@(isolate_dependency_tracked)',
       ],
       'outputs': [
         '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
       ],
       'action': [
         'python',
         '<(DEPTH)/tools/swarm_client/isolate.py',
         '<(test_isolation_mode)',
         '--outdir', '<(test_isolation_outdir)',
         '--variable', 'PRODUCT_DIR', '<(PRODUCT_DIR)',
         '--variable', 'OS', '<(OS)',
         '--result', '<@(_outputs)',
         '--isolate', '<(RULE_INPUT_PATH)',
       ],
       'msvs_cygwin_shell': 0,
     },
   ],
 }