Reorganize Layout Tree Construction code to be its own function

This patch trivially encapsulates Layout Tree Construction
code in its own function -> Element::buildOwnLayout().

No behaviour change is expected from this patch since it is
merely moving code around.

With lots of help from @bugsnash

Design doc: http://bit.ly/29MBWvf

BUG=595137

Review-Url: https://codereview.chromium.org/2118393002
Cr-Commit-Position: refs/heads/master@{#404584}
2 files changed