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This CL implements the writable part of KeyframeEffect's target member.
This brings us into line with the spec, except that Chromium still don't
support CSSPseudoElement and so we only allow setting Elements.

Bug: 813902
Test: external/wpt/web-animations/interfaces/KeyframeEffect/target.html
Test: external/wpt/web-animations/interfaces/KeyframeEffect/idlharness.window.js

Change-Id: I91faf22473e7776a8cad6165ff60c5d9ee47a9a6
Reviewed-on: https://chromium-review.googlesource.com/1157002
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579840}