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If ScopedFD is initialized with some integer, its dtor will close it as a file
descriptor. Because close will cause an error if there is no file descriptor,
this will fail VideoFrame unittest.
To solve this issue, ScopedFD is initialized without anything (i.e. default
ctor) is called.

BUG=chromium:883249
TEST=media_unittest on linux

Change-Id: If89a59d882fd887e4dc443bfd00153e69e336751
Reviewed-on: https://chromium-review.googlesource.com/1223167
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591286}