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Make ClipboardWriter keep FileReaderLoader alive until it's done reading
a Blob, by using SelfKeepAlive<T>.

Previously, ClipboardWriter could be garbage collected unexpectedly
(ex. when the frame detaches). This could cause a use after poison
in the FileReaderLoader, where:
(1) A ClipboardWriter's FileReaderLoader starts reading the input blob
    as an async task.
(2) The ClipboardWriter is destroyed (garbage collected).
(3) The FileReaderLoader completes its async task of reading the input
    blob, and calls ClipboardWriter::StartWrite on the destroyed, owning
    ClipboardWriter.

Additionally, add a "context destroyed" error message when a context
detaches.

Bug: 1142331
Change-Id: I427cd6dc02e773b2d235d45bd9ad8935b575ff71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509033
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827371}