Add browser_tests, content_browsertests and extensions_browsertests to WebKit Linux Oilpan

BUG=

Review URL: https://codereview.chromium.org/1466423005

Cr-Commit-Position: refs/heads/master@{#361325}

testing/buildbot/chromium.webkit.json

@@ -838,6 +838,25 @@
         },
         "test": "blink_platform_unittests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 20
+        },
+        "test": "browser_tests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "content_browsertests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "extensions_browsertests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true