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* luci-go's `isolate archive` requires `-isolate` flag.
* we also enable `-dump-json`, which dumps a JSON file that contains the SHA1 hash of the isolated file.

Tested on the following command:

```
tools/mb/mb.py run -s --no-default-dimensions -d pool chromium.tests -d device_os KTU84P -d device_type hammerhead out/Release/ base_unittests -- --test-filter FakeClass#fakeTest
```

It runs successfully.

Bug: 1062881
Change-Id: I46af6679e43d3af6bfed797485cb485b39235733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2105272Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ye Kuang <yekuang@google.com>
Cr-Commit-Position: refs/heads/master@{#751624}