Disable polygerrit checks on master branch

All master branch FE verification is now handled by LUCI [0] and submits
are gated on the Frontend-Verified submit requirement that LUCI votes to
satisfy. This includes type checking, test running, and linting.

Now that Verified and CodeStyle submit requirements are not needed for
FE-only changes on master branch [1], Jenkins should not waste time
running these builds.

Until LUCI recipes are added for release branches, non-master branches
still need Jenkins verification.

[0]: Change 372706
[1]: Change 373219

Change-Id: Iaefaac42b9ccccc1320531a581871384357ef115
diff --git a/vars/gerritPipeline.groovy b/vars/gerritPipeline.groovy
index 03a6d6a..f33bc47 100644
--- a/vars/gerritPipeline.groovy
+++ b/vars/gerritPipeline.groovy
@@ -142,18 +142,23 @@
         it.startsWith("lib/js") }
     def bazelFiles = changedFiles.findAll { it == "WORKSPACE" || it.endsWith("BUILD") ||
         it.endsWith(".bzl") || it == ".bazelversion" }
-    if(isMerge) {
+    def onlyPolygerritChanges = !polygerritFiles.isEmpty() && changedFiles.size() == polygerritFiles.size() && bazelFiles.isEmpty() && !isMerge
+
+    if (env.GERRIT_BRANCH == "master") {
+        if (onlyPolygerritChanges) {
+            println "Only PolyGerrit UI changes detected on master, skipping all test modes..."
+            Builds.modes = []
+        }
+    } else if (isMerge) {
         println "Merge commit detected, adding 'polygerrit' validation..."
         Builds.modes += "polygerrit"
-    } else if(polygerritFiles.size() > 0) {
-        if(changedFiles.size() == polygerritFiles.size() && bazelFiles.isEmpty()) {
-            println "Only PolyGerrit UI changes detected, skipping other test modes..."
-            Builds.modes = ["polygerrit"]
-        } else {
-            println "PolyGerrit UI changes detected, adding 'polygerrit' validation..."
-            Builds.modes += "polygerrit"
-        }
-    } else if(!bazelFiles.isEmpty()) {
+    } else if (onlyPolygerritChanges)  {
+        println "Only PolyGerrit UI changes detected, skipping other test modes..."
+        Builds.modes = ["polygerrit"]
+    } else if (!polygerritFiles.isEmpty()) {
+        println "PolyGerrit UI changes detected, adding 'polygerrit' validation..."
+        Builds.modes += "polygerrit"
+    } else if (!bazelFiles.isEmpty()) {
         println "Bazel files changes detected, adding 'polygerrit' validation..."
         Builds.modes += "polygerrit"
     }