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Currently VectorBuffer allows the access of buffer[size]. This is
because callers are using buffer[size] to get buffer.end(). In order to
avoid this, use CheckedRandomAccessIterators in VectorBuffer. This
required creating iterator versions for the DestructRange and MoveRange
functions.

CheckedIterator's behavior now depends on the enable_checked_iterators
buildflag: when true, CheckedIterator will CHECK on spatial safety
violations; when false, CheckedIterator will merely DCHECK. This is
temporary, so that binary size regressions on Android can be studied
and explained: the eventual goal is to enable this universally and
remove the buildflag.

BUG=chromium:817982

Change-Id: I2037bdb542c4442011fc9ebffdc50e50ccfafa7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674746Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685387}