Switch trusty -> xenial for linux asan CQ/CI builders

This is step 2 written in
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1307659

Bug: 954450
Change-Id: I0c6b99123befc1de3d70afc7d53c50901521acd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615403Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660731}

infra/config/cr-buildbucket.cfg

@@ -1327,12 +1327,14 @@ buckets {
       name: "Linux ASan LSan Builder"
       mixins: "linux"
       mixins: "memory-ci"
+      mixins: "linux-xenial"
     }
 
     builders {
       name: "Linux ASan LSan Tests (1)"
       mixins: "linux"
       mixins: "memory-ci"
+      mixins: "linux-xenial"
     }
 
     builders {
@@ -3784,6 +3786,7 @@ buckets {
     builders { mixins: "linux-try" name: "linux_chromium_archive_rel_ng" }
     builders {
       mixins: "linux-try"
+      mixins: "linux-xenial"
       mixins: "goma-j150"
       name: "linux_chromium_asan_rel_ng"
     }

testing/buildbot/waterfalls.pyl

@@ -3667,7 +3667,7 @@
       },
       'Linux ASan LSan Tests (1)': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_and_gl_gtests',