[CSSGridLayout] Skip items spanning flex tracks in track sizing algo

Section 10.4 of the specs forces us to ignore items spanning tracks with
flex sizing functions when resolving the content-based track sizing
functions. Items with span < 2 are not affected by this rule (as they will
belong to a single track). This way the algorithm ensures that min-content
and max-content restrictions are fulfilled before distributing the extra
space.

BUG=392200

Review URL: https://codereview.chromium.org/448993002

git-svn-id: svn://svn.chromium.org/blink/trunk@183807 bbb929c8-8fbe-4397-9dbb-9b2b20218538