Pass absolute paths to wpt lint in PRESUBMIT.py

An upstream change in wpt lint interprets relative paths as relative to
CWD instead of the (WPT) repo root, which breaks PRESUBMIT.py.

Use absolute paths instead to avoid any ambiguity.

Bug: 819349
Change-Id: Idf57e048c30d3143e55ffebfc95df0f774ae4e8f
Reviewed-on: https://chromium-review.googlesource.com/951755Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541280}

third_party/WebKit/LayoutTests/external/PRESUBMIT.py

@@ -20,7 +20,7 @@ def _LintWPT(input_api, output_api):
     for f in input_api.AffectedFiles():
         abs_path = f.AbsoluteLocalPath()
         if abs_path.startswith(wpt_path):
-            paths_in_wpt.append(abs_path[len(wpt_path) + 1:])
+            paths_in_wpt.append(abs_path)
 
     # If there are changes in LayoutTests/external that aren't in wpt, e.g.
     # changes to wpt_automation or this presubmit script, then we can return