Add virtual destructor to SignalReceiver.

class angle::SignalReceiver has virtual functions
and accessible non-virtual destructor.
This generates a compile error.

Change-Id: Iced2ee94ad9268ab5c1d00ab336ab2cc98385afc
Reviewed-on: https://chromium-review.googlesource.com/351930Reviewed-by: Sami Väisänen <svaisanen@nvidia.com>
Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>