Fail if an isolate entry refers to a missing file.

This might have been useful before while trying to get the system running. But now we have swarming running on Linux/Mac/Windows trybots, so it's not so useful. It's also confusing when tests fail on swarming machines because an isolate file referred to a binary that wasn't in the dependencies.

BUG=414808

Review URL: https://codereview.chromium.org/575363002

Cr-Original-Commit-Position: refs/heads/master@{#295514}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6d2b3cd041bcded84f6f82f1fbac2348e2d7a9df

common.gypi

@@ -591,7 +591,7 @@
       'test_isolation_outdir%': '',
       # True if isolate should fail if the isolate files refer to files
       # that are missing.
-      'test_isolation_fail_on_missing': 0,
+      'test_isolation_fail_on_missing': 1,
 
       'wix_path%': '<(DEPTH)/third_party/wix',