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This patch adds layer list logging. To use, start content shell with:
  --vmodule=layer_tree_host=3

Sample output:
  layer id 11
    element_id: (0)
    bounds: 800x600
    opacity: 1
    position: 0.000000,0.000000
    draws_content: 1
    scrollable: 0
    contents_opaque: 1
    transform_tree_index: 2
    clip_tree_index: 1
    effect_tree_index: 1
    scroll_tree_index: 1
  layer id 14
    ...

There is currently no logging for cc::Layer so a selection of important
properties has been dumped.

Bug: 854200
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I938492167619c262976083291bd6db9b4a39fef5
Reviewed-on: https://chromium-review.googlesource.com/1175855
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583755}