ProgramD3D::GetExecutableTask is a d3d::Context type itself so that errors can
be recorded in a thread-safe maner.  Use the task context when loading program
binaries in ProgramD3D.

Fix up incorrect casts in RendererD3D::loadExecutable in both D3D9 and D3D11
backends to stop incorrectly assuming the context type.

BUG=962439

Change-Id: I5b29372a7254f709e1bbb67ee322ef4109f73e48
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1630294Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>