Add 'UBSanVptr Linux' buildbot (src part).

Right now, we have a Clang ToT UBsan vptr buildbot:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinuxUBSanVptr

Unfortunately, it's very slow, as it does not use goma
(Clang ToT could not do that). Not only we want to run ubsan vptr
tests more often, we also want to have a trybot (to be added later)
for the same configuration. At some point (closer to CFI launch on Linux),
the trybot will become mandatory for the commit queue, as we would like
to fail early.

BUG=588196

Review URL: https://codereview.chromium.org/1905553004

Cr-Commit-Position: refs/heads/master@{#388651}

tools/mb/mb_config.pyl

@@ -205,6 +205,7 @@
       'Linux ARM': 'swarming_gyp_release_bot_arm',
       'Site Isolation Linux': 'gn_release_trybot',
       'Site Isolation Win': 'gyp_release_trybot_x64',
+      'UBSanVptr Linux': 'gn_ubsan_vptr_release_bot',
       'Vista Tests (dbg)(1)': 'none',
       'Vista Tests (dbg)(2)': 'none',
       'WebKit Linux - TraceWrappables': 'swarming_gn_release_bot_x64',
@@ -1162,6 +1163,10 @@
       'gn', 'release_trybot', 'x86',
     ],
 
+    'gn_ubsan_vptr_release_bot': [
+      'gn', 'ubsan_vptr', 'release_bot',
+    ],
+
     'gyp_asan_lsan_edge_fuzzer_debug_bot': [
       'gyp', 'asan', 'lsan', 'edge', 'fuzzer', 'debug_bot',
     ],