Scroll performance improved in multiline TextEdit

Measuring line widths, glyph by glyph slows down the scrolling
process for long text (for some reason, width measure efficiency
is affectedi by text length, maybe because the whole text has to
be passed to JNI layers).

This optimization avoids this computation in the case where there
is no possible horizontal scroll.

This is a cherry pick of 145957 into ICS-MR1

Change-Id: I2082e3d0eedace1a86122a03e4b21f90f3bc8522