skia_dm rendering through HWUI was hanging after roughly 300 tests.
logcat reports some process was unable to get any file descriptors.
When we migrated TestWindowContext into HWUI and started using PIMPL
I didn't clean up the implementation struct in the destructor. Doing
so solved the hang for me.
My guess was that BufferQueue was being leaked, and that gralloc is
backed by file descriptors, but some research suggests that Android
ought to be able to handle far more than 1k fds (ulimit -Hn returns
4096 on this device).
R=djsollen@google.com
Change-Id: I2cd9f8945cee9b22f838002e1ad687d5fe29cb97
58862c9f6f