[test] Run webkit tests as part of default testing on the bots.

The test suite is ran in 60% of the bots anyway and the
step is very short. For swarming, it's better to run this
together in one step as each step triggers a different bot.

BUG=chromium:535160
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1413023002

Cr-Commit-Position: refs/heads/master@{#31360}

tools/run-tests.py

@@ -60,6 +60,15 @@ ARCH_GUESS = utils.DefaultArch()
 # expected runtimes (suites with slow test cases first). These groups are
 # invoked in seperate steps on the bots.
 TEST_MAP = {
+  "bot_default": [
+    "mjsunit",
+    "cctest",
+    "webkit",
+    "message",
+    "preparser",
+    "intl",
+    "unittests",
+  ],
   "default": [
     "mjsunit",
     "cctest",