ozone: fix crash when running video decode unittests.

In GLSurfaceOzoneSurfacelessSurfaceImpl::Destroy, if the previous
context was NULL, do not make it current.

BUG=chromium:602921
TEST=Run vda unittests on oak.

Review URL: https://codereview.chromium.org/1887563002

Cr-Commit-Position: refs/heads/master@{#386988}
bool GLSurface::InitializeOneOffInternal() {
  switch (GetGLImplementation()) {
    case kGLImplementationEGLGLES2:
      if (!GLSurfaceEGL::InitializeOneOff()) {
        LOG(ERROR) << "GLSurfaceEGL::InitializeOneOff failed.";
        return false;
      }

      return true;
    case kGLImplementationOSMesaGL:
    case kGLImplementationMockGL:
      return true;
    default:
      return false;
  }
}

bool GLSurface::InitializeOneOffInternal() {
  switch (GetGLImplementation()) {
    case kGLImplementationEGLGLES2:
      if (!GLSurfaceEGL::InitializeOneOff()) {
        LOG(ERROR) << "GLSurfaceEGL::InitializeOneOff failed.";
        return false;
      }

      return true;
    case kGLImplementationOSMesaGL:
    case kGLImplementationMockGL:
      return true;
    default:
      return false;
  }
}
