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}
3 files changed