Add new isolation mode "prepare".

build/isolate.gypi

Index: build/isolate.gypi
diff --git a/build/isolate.gypi b/build/isolate.gypi
index b0b2ca05679a3c5135199a60ad8a1b4cef955b62..907fbb5c2d96befc821ec49daff95ae26bec3102 100644
--- a/build/isolate.gypi
+++ b/build/isolate.gypi
@@ -63,14 +63,12 @@
         # the switch-over to running tests on Swarm is completed.
         #'<@(isolate_dependency_tracked)',
       ],
-      'outputs': [
-        '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
-      ],
+      'outputs': [],
       'action': [
         'python',
         '<(DEPTH)/tools/isolate_driver.py',
         '<(test_isolation_mode)',
-        '--isolated', '<@(_outputs)',
+        '--isolated', '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
         '--isolate', '<(RULE_INPUT_PATH)',
 
         # Variables should use the -V FOO=<(FOO) form so frequent values,
@@ -118,6 +116,15 @@
         ['test_isolation_fail_on_missing == 0', {
           'action': ['--ignore_broken_items'],
         }],
+        ["test_isolation_mode == 'prepare'", {
+          'outputs': [
+            '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated.gen.json',
+          ],
+        }, {
+          'outputs': [
+            '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
+          ],
+        }],
       ],
     },
   ],