V4L2ImageProcessor: get the number of planes from the driver.
V4L2ImageProcessor: get the number of planes from the driver.

media::VideoFrame::NumPlanes returns the number of logical planes.
V4L2 API takes the number of physical planes. Y, U, V, or UV are
logical planes. Physical planes are non-continuous buffers. For
example, V4L2_PIX_FMT_NV12 has two logical planes and one physical
plane. V4L2_PIX_FMT_NV12M has two logical planes and two physical
planes.

Driver returns the number of planes from VIDIOC_S_FMT. We should
use it and not media::VideoFrame::NumPlanes.

BUG=chrome-os-partner:46210
TEST=Run VEA unittest on pit. Run apprtc loopback and Hangout.

Review URL: https://codereview.chromium.org/1915833005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389711}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk