Add first letter test when first letter is inside a floated block.

This CL adds a test for the RenderBlock::computePreferredLogicalWidths
call to updateFirstLetter. In the case where we don't call
updateFirstLetter we will incorrectly draw the word Foo flowing out
of the enclosing border.

BUG=391288

Review URL: https://codereview.chromium.org/373503002

git-svn-id: svn://svn.chromium.org/blink/trunk@178584 bbb929c8-8fbe-4397-9dbb-9b2b20218538