Should be easy to fix. We basically don't track those changes when we check whether or not the new style requires a new layout.

Yeah sure, I'll take this one.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/468f3c355c499087911dbb34c8d2ba6d9a014cff

commit 468f3c355c499087911dbb34c8d2ba6d9a014cff
Author: rego <rego@igalia.com>
Date: Thu Oct 13 19:16:27 2016

[css-grid] Recompute track sizes if fit-content() argument is modified

When fit-content() was implemented a new attribute
m_fitContentTrackBreadth was added to GridTrackSize.
However we forgot to include it in the == operator implementation,
so we were missing the changes on that value.

Now if you change the argument of a fit-content() track,
the track size is properly recomputed.

BUG= 654712 
TEST=fast/css-grid-layout/grid-change-fit-content-argument.html

Review-Url: https://codereview.chromium.org/2420673002
Cr-Commit-Position: refs/heads/master@{#425122}

[add] https://crrev.com/468f3c355c499087911dbb34c8d2ba6d9a014cff/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-change-fit-content-argument-expected.txt
[add] https://crrev.com/468f3c355c499087911dbb34c8d2ba6d9a014cff/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-change-fit-content-argument.html
[modify] https://crrev.com/468f3c355c499087911dbb34c8d2ba6d9a014cff/third_party/WebKit/Source/core/style/GridTrackSize.h