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The current system truncates the file at various points. This means that
if chrome happens to crash during that time, the user won't have
anything to restore. The fix will be to not truncate the file, and
instead create a new one rather than truncating. As a result of this,
snapshotting and non-snapshotting will effectively become the same. This
patch combines the two classes. This patch doesn't actually change the
logic yet, it just combines the two.

BUG=648266
TEST=covered by tests

Change-Id: I90a0483c265c64fa3343bce080d7061c1074b879
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605573Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840260}