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This change appends the pinned depot_tools in Chromium checkout (at
//src/third_party/depot_tools) to PATH so that the wpt_{import,export}
scripts always work as long as we have a `gclient sync`ed checkout.

The main reason for the change is to fix the scripts on LUCI, where
depot_tools is no longer in PATH by default.

Bug: 803111
Change-Id: I9242befa3dbc9e735ae3a618dcd5b8f5eff5579d
Reviewed-on: https://chromium-review.googlesource.com/1252532Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595457}