Fieldset should allow whitespaces even though LayoutFlexibleBox doesn't.
Restore the correct behaviour for canHaveWhitespaceChildren to return
true for <fieldset> because it was changed when LayoutFieldSet was
switched to use LayoutFlexibleBox.
Revert some incorrectly rebaselined tests from
https://crrev.com/419570.
For example: it's expected to have a trailing empty LayoutText for a
fieldset layout object. See fieldset-align-expected.txt
BUG=
648209
Committed:
https://crrev.com/1833c7446ae72fa3437e73e9323c87f89b034c2f
Cr-Commit-Position: refs/heads/master@{#421911}