SideBySide2: Fix alignment for long insertion/deletion blocks

In commit 1dbc53126cafc0f553b709 the line-height was changed to
"normal", allowing for taller text lines. Padding for insertion
or deletion blocks was no longer tall enough, which caused the
two sides to stop aligning.

Correct this by computing the line height on the fly after the
UI is displayed and the browser was able to compute font metrics
to a sampling of 10 lines of text.

Use the guessed value from a prior rendering for the new one,
reducing any reflow that has to occur upon viewing another
file in the same application session.

Chrome on Linux is showing the lines are 15px tall, so use
that as the current default guess, reducing initial reflow
for any platform that has this font height.

Bug: issue 2970
Change-Id: Ic5f7dbbd1cc3582388cfcdf6faf9dca1f7a85fa0
(cherry picked from commit 941d32cff210e3bec4e114ea35822b017faf18f1)