[merge to m53] Fix getComputedStyle for non-box-layout elements

This fixes a crash introduced by https://codereview.chromium.org/1826423003/
Code that treated a layout object as a Box layout were not properly
guarded by isBox.  This caused a crash when calling getComputedStyle on
positioned non-box elements, e.g. <ruby>.

R=mstensho@opera.com
BUG=610986

Review-Url: https://codereview.chromium.org/2102843002
Cr-Commit-Position: refs/heads/master@{#404336}
(cherry picked from commit 86d16a615d6e9abf6d991f430b496c26a4271d10)

Review URL: https://codereview.chromium.org/2177243004 .

Cr-Commit-Position: refs/branch-heads/2785@{#347}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}
3 files changed