The LayoutTest expectations on Android for many tests under wpt/webmessaging are trivial:

layer at (0,0) size 800x600
  LayoutView at (0,0) size 800x600
layer at (0,0) size 800x600
  LayoutBlockFlow {HTML} at (0,0) size 800x600
    LayoutBlockFlow {BODY} at (8,8) size 784x584

These are testharness.js tests so they shouldn't be triggering a dump of the layout tree.

https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/platform/android/external/wpt/webmessaging/?type=cs&sq=package:chromium