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Due to performance regressions I've reduced the percentage of processes
GWP-ASan is enabled for. As a result, I've been able to proportionally
increase the amount of memory we use per process. As a result, the
previous maximum metadata of 255 no longer sufficies. Increase
MetadataIdx to be a 16-bit type and increase the maximum metadata to
something more reasonable.

Change-Id: If031dec3732ccf3ba69bdafdca2b03cb274bd9ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761477
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688424}