Add webapk_client_junit_tests to bots.

Please refer to CL (https://codereview.chromium.org/1971773002/) that
adds the webapk_client_junit_tests target.

BUG=609122

Review-Url: https://codereview.chromium.org/1978863002
Cr-Commit-Position: refs/heads/master@{#393708}
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index 7e48718..ea08ccbb 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -193,6 +193,9 @@
       },
       {
         "test": "ui_junit_tests"
+      },
+      {
+        "test": "webapk_client_junit_tests"
       }
     ]
   },
@@ -354,6 +357,9 @@
       },
       {
         "test": "ui_junit_tests"
+      },
+      {
+        "test": "webapk_client_junit_tests"
       }
     ],
     "scripts": [
@@ -492,6 +498,9 @@
       },
       {
         "test": "ui_junit_tests"
+      },
+      {
+        "test": "webapk_client_junit_tests"
       }
     ],
     "scripts": [