Update a path to layout tests in blink/renderer/platform/BUILD.gn

NOTRY=true
NOTREECHECKS=true

Bug: 830180
Change-Id: Ib02e205eb54d19e7e2a5b7531aea0c379dba2703
Reviewed-on: https://chromium-review.googlesource.com/1001252
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549065}

third_party/blink/renderer/platform/BUILD.gn

@@ -2055,7 +2055,7 @@ group("blink_platform_unittests_data") {
 
     # Required by some image decoder tests.
     "image-decoders/testing/",
-    "../../LayoutTests/images/resources/",
+    "//third_party/WebKit/LayoutTests/images/resources/",
   ]
 }