Turbolizer only remembered the expansion state of the panes,
but not their widths. This CL remembers the relative widths,
and restores them upon reload. This is also useful when the
size of the Turbolizer window changes.

Change-Id: I0fd81c1266bfbddded86da16e2241420cdf73f4e
Notry: true
Bug: v8:7327
Reviewed-on: https://chromium-review.googlesource.com/c/1396421
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58575}