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This will update the Pixel2 bots to use the MonoChrome6432.aab which is a 64bit
browser. In making this change a new 'exact_browser' option is added to
tester_config in perf_data_generator to allow specifying a particular binary to
be run rather than guessing.

Reland of: https://chromium-review.googlesource.com/c/chromium/src/+/1511455

TBR=crouleau@chromium.org
BUG=934251

Change-Id: I21bf7ae8e42f0e4c127748eeecc58a8af34334a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1587563Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654862}