Make ElementResolveContext hold a ref to the root element's ComputedStyle

Bug: 947029
Change-Id: I167aad139398a808d170f39e72c5eef57f142e11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556653
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650001}