Plug memory leak.

Bug #6196903

Whenever a memory flush happens, the GL renderer discards some or all of its
font caches. Each font cache holds an array of vertex indices that was
initially designed to have the same life cycle as the process. This changed
when memory flushes were introduced but this array was never taken care of
in the destructor.

Change-Id: Ief124f609ea55b671c0a9b43637d9e013629ebaa