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This patch adds an SkRect for bounds to compositing_display_item and
uses it for setting the canvas layer bounds. With this patch and the
associated blink-side patch [1] we pass the following additional
layout tests:
LayoutTests/svg/zoom/page/zoom-mask-with-percentages.svg
LayoutTests/svg/transforms/text-with-mask-with-svg-transform.svg
LayoutTests/svg/custom/absolute-sized-content-with-resources.xhtml
LayoutTests/svg/custom/mask-excessive-malloc.svg
LayoutTests/svg/batik/masking/maskRegions.svg

[1] https://codereview.chromium.org/960483002

Review URL: https://codereview.chromium.org/952123002

Cr-Commit-Position: refs/heads/master@{#318127}