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The forward declaration in in layer_tree_host_impl.h seems to be in
alphabetic order where one of them isn't.

This CL corrects the order, and also removes a forward-declaration for
a struct which isn't needed.

Bug: None
Change-Id: I275a1af8392f20ec023a69107aa508b2a29b3d55
Reviewed-on: https://chromium-review.googlesource.com/c/1352515Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611538}