Replace integer math with self-documenting duration math.

Change-Id: If1026afabd6da9f61bb0f6371d7d521271b6eefe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303038
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789203}

infra/config/subprojects/chromium/master-only/ci.star

@@ -2955,7 +2955,7 @@ ci.memory_builder(
     cores = 32,
     # TODO(https://crbug.com/919430) Remove the larger timeout once compile
     # times have been brought down to reasonable level
-    execution_timeout = time.hour * 9 / 2,  # 4.5 (can't multiply float * duration)
+    execution_timeout = 4 * time.hour + 30 * time.minute,
     tree_closing = False,
 )
 

infra/config/subprojects/chromium/master-only/try.star

@@ -708,7 +708,7 @@ try_.chromium_win_builder(
 
 try_.chromium_win_builder(
     name = 'win7-rel',
-    execution_timeout = time.hour * 9 / 2,  # 4.5 (can't multiply float * duration)
+    execution_timeout = 4 * time.hour + 30 * time.minute,
     goma_jobs = goma.jobs.J300,
     ssd = True,
 )