text-overflow on button should have effect

RenderButton is derived from RenderFlexibleBox.
In https://codereview.chromium.org/23449034 its mentioned that
text-overflow on flexbox should have no effect. But in Firefox
button text-overflow:ellipsis have a effect.
I have added a exception for Renderbutton by returning true
for canCollapseAnonymousBlockChild().

BUG=107014, 319894

Review URL: https://codereview.chromium.org/143983013

git-svn-id: svn://svn.chromium.org/blink/trunk@165720 bbb929c8-8fbe-4397-9dbb-9b2b20218538