`UseAutoISize` flag is buggy when used on table flex items because it
never propagates to inner table frame.

Luckily, we can fix it by replacing the flag with StyleSizeOverrides
emplacing an 'auto' mStyleISize, because when computing the size, the
size overrides already propagates from table wrapper to inner table via
the following path:

  `nsTableWrapperFrame::ComputeSize()` [1] ->
  `nsTableWrapperFrame::ComputeAutoSize()` ->
  `nsTableWrapperFrame::InnerTableShrinkWrapISize()` ->
  `nsTableFrame::ComputeSize()`.

Part 3 is going to propagate the size overrides to inner table in
`nsTableWrapperFrame::CreateReflowInputForInnerTable()` during reflow.

This patch fixes the content size suggestion for table flex items.
Combining this patch with Part 3, we can fix those reftests in Part 4.

[1] In this patch, the table wrapper is still using nsContainerFrame's
ComputeSize(), but Part 3 is going to override it.

Differential Revision: https://phabricator.services.mozilla.com/D103438