[css-flexbox] Make sure to layout overflow:auto elements

For overflow: auto, we need to layout before running the flex algorithm
if layout is dirty, so we can up-to-date scrollbar information, because
scrollbar size is included in the intrinsic size.

(cherry picked from commit 408ad4e0f177e8f6116963387576edd3f33c6e45)

Bug: 972227
Change-Id: I58b5c0590575c5babc505b2927d8464f3f540775
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652198
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#668199}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660788
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#315}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

• third_party/blink/renderer/core/layout/layout_flexible_box.cc[diff]
• third_party/blink/web_tests/TestExpectations[diff]
• third_party/blink/web_tests/external/wpt/css/css-flexbox/overflow-auto-001.html[Added - diff]