Add new isolation mode "prepare".

In that mode isolate_driver.py will collect all arguments needed for
isolate.py invocation into *.isolated.gen.json file, but won't actually
run the isolation itself.

It's a feature of isolate_driver.py. Isolate client itself knows nothing about
this mode.

BUG=389227
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/555553002

Cr-Original-Commit-Position: refs/heads/master@{#293885}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8556ebdc21ada4753aa6798a240c83101f212dcf

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',
+          ],
+        }],
       ],
     },
   ],