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Ensure that a format is valid before creating a placeholder Blob, so
that clipboard.read() no longer returns nullptr blobs. In particular,
filter out custom/pickled formats not supported by the Clipboard API

Previously, custom/pickled formats not supported by the Clipboard API,
but exposed via blink::SystemClipboard, would create nullptr Blobs when
read via clipboard.read(), and return them in the output ClipboardItems.
This output ClipboardItem could then be written via clipboard.write(),
and cause a crash.

Bug: 1153361
Change-Id: Ib57ccca0e79adb09ee51e64c324c32283c788cbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568818
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833501}