Create BUILD.gn files for //media/video.

Create new BUILD.gn file for media/video and moved respective
media component changes from media/filters/BUILD.gn file to new
BUILD.gn file.

BUG=613033

Signed-off-by: Suchit Agrawal <a.suchit@samsung.com>
Change-Id: Iaa2ff8c16d697ede5e8e249fbc01c058eaef6d78
Reviewed-on: https://chromium-review.googlesource.com/569528
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: SUCHIT AGRAWAL <a.suchit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487055}
2 files changed