Mark Win7 GPU bots slow.

These bots had a 3h timeout specified manually, where the rest of the
bots on the chromium.gpu.fyi waterfall which use the win-gpu-fyi-ci
mixin pick up the gpu-slow-bot specification automatically, which
provides a 6h timeout. Remove the manual execution_timeout_secs for
these bots while Issue 917211 is being investigated.

Bug: 928303
Change-Id: I8c4bd12e8b06c6391a5ed2ea4f4d15ab16111e02
Reviewed-on: https://chromium-review.googlesource.com/c/1453058
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628918}
1 file changed