Correctly compute the number of bytes written by each op.

Bug #6157792

Previously, DisplayListRenderer would compute the number of bytes
written after a drawing op by looking at the difference between
the start pointer of the command stream and the end pointer of
the command stream. The SkWriter class used to record the commands
stream allocates blocks of storage which would cause a crash when
a command spanned two blocks.

Change-Id: I4d79d3feeb6d72d9d4e6ab05ecebd72d004be56c