vp8_ratectrl_rtc_test.cc: ensure frame_type is initialized

vp8_ratectrl_rtc_test.cc: ensure frame_type is initialized

this fixes a valgrind failure:
==1095597== Conditional jump or move depends on uninitialised value(s)
==1095597==    at 0x12E0CC: (anonymous
namespace)::Vp8RcInterfaceTest::PreEncodeFrameHook(libvpx_test::VideoSource*,
libvpx_test::  > Encoder*) (vp8_ratectrl_rtc_test.cc:131)
==1095597==    by 0x1255A9:
libvpx_test::EncoderTest::RunLoop(libvpx_test::VideoSource*)
(encode_test_driver.cc:205)

Bug: webm:1776
Change-Id: Id3b40f62573ee513e79c74b6315c71b6ecd22c9a
Fixed: webm:1776