Fix buildbreak on Linux GCC
Fix buildbreak on Linux GCC

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/351930
Reviewed-by: Sami Väisänen <6d8a3c450232f0013fe768a43ce20fd9c25617e8@nvidia.com>
Commit-Queue: Sami Väisänen <6d8a3c450232f0013fe768a43ce20fd9c25617e8@nvidia.com>

MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle,ppy/angle,ecoal95/angle,ecoal95/angle,ecoal95/angle,domokit/waterfall,ppy/angle,ppy/angle,ppy/angle,domokit/waterfall,MSOpenTech/angle,ecoal95/angle,MSOpenTech/angle