Reland "Ramp up cast_shell_linux on LUCI to 100%."

This is a reland of d553b24a

Original change's description:
> Ramp up cast_shell_linux on LUCI to 100%.
> 
> Bug: 731556
> Change-Id: Id416d395958f0b3529816f17ca0d35203f863065
> Reviewed-on: https://chromium-review.googlesource.com/950023
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#541215}

Bug: 731556
Change-Id: I58d5362dc01773a700c602213cab322444701bba
Reviewed-on: https://chromium-review.googlesource.com/952442Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541242}

infra/config/branch/cq.cfg

@@ -79,7 +79,7 @@ verifiers {
       name: "master.tryserver.chromium.linux"
       builders {
         name: "cast_shell_linux"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 10 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 100 }
       }
       builders {
         name: "chromium_presubmit"