[CSS Shapes] Rounded Insets Let Content Overlap Shape

Using LengthSize to FloatSize conversion from LengthSize.h lead to miscalculated
inset border radius, when the border radius was defined by percentages. This patch
fixes the behavior and removes the incorrect conversion.

BUG=341584

Review URL: https://codereview.chromium.org/157083002

git-svn-id: svn://svn.chromium.org/blink/trunk@166986 bbb929c8-8fbe-4397-9dbb-9b2b20218538