[css-flex/tables] Stop flex from shrinking tables below their min height

Tables can't have height less than their intrinsic block size.
Otherwise, cells would overflow the table, which breaks a table
invariant.

FlexNG was assigning tables heights that are too short. Legacy tables
ignored FlexNG and assigned themselves a proper height, so we never
noticed this bug. TablesNG was trying to obey flex's imposed height,
which brought this FlexNG bug to light.

Now FlexNG always assigns a block size greater than or equal to the
table's intrinsic block size when it is laid out with whatever final
inline size the flexbox gives (stretched or unstretched).

There is no spec requiring this in the block direction, even though
Firefox and legacy Blink both do it, and there is a spec requirement for
the inline direction. So the tests are marked tentative.

Bug: 1181403
Change-Id: I538188d2749d0c1e0a277068c59fbb26759b1756
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2810520
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#870270}