Refactors Vp9UncompressedHeaderParser.

Biggest change is a new helper class used to read data from the
bitstream and then pass the result to a function if reading was
successful. There's also helper to do if/else flow based on the read
values. This avoids a bunch of temporaries and in my view makes the
code esaier to read.

For example, this block:

uint32_t bit;
RETURN_FALSE_IF_ERROR(br->ReadBits(&bit, 1));
if (bit) {
  RETURN_FALSE_IF_ERROR(br->ConsumeBits(7));
}

...is now written as:

RETURN_IF_FALSE(
    br->IfNextBoolean([br] { return br->ConsumeBits(7); }));

In addition, we parse and put a few extra things in FrameInfo:
show_existing_frame, is_keyframe, and base_qp.

Bug: webrtc:12354
Change-Id: Ia0b707b223a1afe0a4521ce2b995437d41243c06
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215239
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33776}