Update default trybots.

Add new gn trybot and remove nosnap dbg for being too slow.
It can be used on demand.

BUG=chromium:353487
LOG=n

Review URL: https://codereview.chromium.org/743643002

Cr-Commit-Position: refs/heads/master@{#25419}

PRESUBMIT.py

@@ -244,11 +244,11 @@ def GetPreferredTryMasters(project, change):
       'v8_linux_rel': set(['defaulttests']),
       'v8_linux_dbg': set(['defaulttests']),
       'v8_linux_nosnap_rel': set(['defaulttests']),
-      'v8_linux_nosnap_dbg': set(['defaulttests']),
       'v8_linux64_rel': set(['defaulttests']),
       'v8_linux_arm_dbg': set(['defaulttests']),
       'v8_linux_arm64_rel': set(['defaulttests']),
       'v8_linux_layout_dbg': set(['defaulttests']),
+      'v8_linux_chromium_gn_rel': set(['defaulttests']),
       'v8_mac_rel': set(['defaulttests']),
       'v8_win_rel': set(['defaulttests']),
       'v8_win64_compile_rel': set(['defaulttests']),