switch to use xenial for test from Linux Future builder

Each builder has around 10 hour hourly resource usage in p99.
http://shortn/_gZ2rCCbPNR

So I expect this won't add much burden to existing xenial pool.
Current usage is in
https://datastudio.google.com/s/uf5Kh6NZ8XA
And we have 3400 xenial bots in MTV time now.

Bug: 954890
Change-Id: I18d8e0cd6e8609f4bb924e348555c8d38d14d1bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730492Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683170}

testing/buildbot/waterfalls.pyl

@@ -4170,7 +4170,7 @@
     'machines': {
       'Linux - Future': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'client_v8_chromium_gtests',
@@ -4179,7 +4179,7 @@
       },
       'Linux - Future (dbg)': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'client_v8_chromium_gtests',