Fix media_unittests failures with passthrough

With passthrough command decoder, it will use glClear() to clear
every textures without initial data. However some textures can not
be attached to framebuffer. It will cause problem. Fix the problem
by providing initial data.

For example: some media unittests will create shared images with L8
format and then use glTextSubImage2D() to upload the content. The
passthrough command decoder will try to clear the texture with
glClear(), but L8 texture is not renderable.

Bug: 1361625
Change-Id: I1f5c7e1dc1abb03bbd515bc79cd83c25f14a3cc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3883862


Reviewed-by: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Auto-Submit: Peng Huang <penghuang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045339}