infra: linux-goma-rbe-staging-rel experiment 20% -> 50%

Since increasing to 20% on 2018-11-16, there is still no saturation of
bots: https://screenshot.googleplex.com/VRSaKSXnSvs.png

Bug: 855319
Change-Id: I417a8a3ac573c8b728eaca322186eda9b0b8602b
Reviewed-on: https://chromium-review.googlesource.com/c/1341442
Commit-Queue: Simon Que <sque@chromium.org>
Reviewed-by: smut <smut@google.com>
Cr-Commit-Position: refs/heads/master@{#609882}

docs/infra/cq_builders.md

@@ -257,7 +257,7 @@ as required builders.
 
   https://crbug.com/855319
 
-  * Experimental percentage: 20
+  * Experimental percentage: 50
 
 * [win-libfuzzer-asan-rel](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win-libfuzzer-asan-rel) ([`cq.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:cq.cfg+win-libfuzzer-asan-rel)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+win-libfuzzer-asan-rel))
 

infra/config/branch/cq.cfg

@@ -236,7 +236,7 @@ verifiers {
       # https://crbug.com/855319
       builders {
         name: "linux-goma-rbe-staging-rel"
-        experiment_percentage: 20
+        experiment_percentage: 50
       }
       builders {
         name: "win-libfuzzer-asan-rel"