Freeze scrollbars for relayout if overflow:auto added scrollbars.

Freeze scrollbars for relayout if overflow:auto added scrollbars.

Under very specific circumstances, LayoutBlockFlow::layoutBlock will
loop infinitely in this stanza:

    while (!done)
        done = layoutBlockFlow(...);

... due to fluctuations in the existence of overflow:auto scrollbars
in descendants.  Any layout type that does multi-pass layout is
especially susceptible to this, hence this bug affecting flexbox,
and another recent bug affecting tables: crbug.com/64635.

This fix supercedes the existing fix for the table bug, and it
should fix any other variations of this bug involving multi-pass
layout and overflow:auto.

Cherry-picked from https://codereview.chromium.org/2345103002

BUG=644450
TBR=cbiesinger@chromium.org,eae@chromium.org
NOTRY=true
NOPRESUBMIT=true

[szager1, 2016-09-23 04:50:01]

The CQ bit was checked by szager@chromium.org

[commit-bot: I haz the power, 2016-09-23 04:50:16]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-23 04:54:36]

Committed patchset #1 (id:1)