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We see a large fraction of unmapped samples with cycles precise.
Disable sampling on cycles precise while we investigate the root
cause of unmapped samples.

BUG=b:170558957,b:168312716

Change-Id: Ie566e8e870e0bebc8db8355222f4cbf8cc61e055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471579Reviewed-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817107}