[testing] use xenial for test from clang tot builders

ToT builders use at most 43 hours hourly resource usage in recent 30
days
https://screenshot.googleplex.com/wzLX3JisHDU (from http://shortn/_N0m7MLW0kg)

ToTLinuxOfficial has only 3~4 hours hourly resource usage in internal pool.
http://shortn/_EPucI9F8nx

I think this fits to current buffer in Chrome pool
that we have 3250 xenial bots in MTV time now.
https://datastudio.google.com/s/uf5Kh6NZ8XA

Bug: 954450
Change-Id: If9e68bfe201c21e473a17250d823d03a4ff7c851
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732432
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683612}

testing/buildbot/waterfalls.pyl

@@ -711,7 +711,7 @@
     'machines': {
       'CFI Linux ToT': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_and_gl_gtests',
@@ -807,7 +807,7 @@
       },
       'ToTLinux': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_gtests',
@@ -820,7 +820,7 @@
       },
       'ToTLinuxASan': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'all',
@@ -834,7 +834,7 @@
       },
       'ToTLinuxMSan': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'all',
@@ -846,7 +846,7 @@
       # Used on an internal builder.
       'ToTLinuxOfficial': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_gtests',
@@ -854,7 +854,7 @@
       },
       'ToTLinuxTSan': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'all',
@@ -865,7 +865,7 @@
       },
       'ToTLinuxThinLTO': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_gtests',
@@ -873,7 +873,7 @@
       },
       'ToTLinuxUBSanVptr': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'all',