FileVideoCaptureDevice: better error mgmt if !File.IsValid()

Apparently on Windows platforms there's a small but nonzero crash rate
due to CHECK(file.IsValid()) in FileVideoCaptureDevice::OpenFileForRead().

Do not CHECK() there, instead let the callers deal with the problem,
so that there is no crash either (capture won't work of course).

BUG=466408

Review URL: https://codereview.chromium.org/1017633004

Cr-Commit-Position: refs/heads/master@{#321139}
2 files changed