Multiple segment shape-inside polygon causes assertion failed with shape-outside iframe

When we positioned a float inside a shape-inside in RenderBlockFlow::computeLogicalLocationForFloat,
we modified the left and right offsets only when we had a shape exactly with 1 segment. Lack of setting
the left/right offset for multiple-segments cases lead the float positioned out of the boundaries of the
shape, which lead to an assert in the calculation of segmentranges (LineBreaker::nextLineBreak).

This patch modifies the code to set the left/right offsets for the float to the first segment in a multiple-
segment shape case in shape-inside. The correct multiple-segment support for shape-inside requires more
attention, and it's out of the scope of this patch, so I didn't do any additional changes on the code.

BUG=344359

Review URL: https://codereview.chromium.org/176963010

git-svn-id: svn://svn.chromium.org/blink/trunk@168011 bbb929c8-8fbe-4397-9dbb-9b2b20218538