Reland "Migrate linux-rel to run on bionic bots"

This is a reland of 9490266

The error happened in crbug/1203700 is not blocker for this
migration(crrev.com/c/2857702).

Original change's description:
> Migrate linux-rel to run on bionic bots
>
> BUG=1199425
> TEST=main.star
>
> Change-Id: Iff8555a0d32527dc7671da6784f988a15fed7501
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2853834
> Commit-Queue: Xinan Lin <linxinan@chromium.org>
> Reviewed-by: Ben Pastene <bpastene@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#876731}

Bug: 1199425
Change-Id: Id40885820db2de71b9333ac9b750abfcbab233f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2858766
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#877513}

infra/config/generated/cr-buildbucket.cfg

@@ -46891,7 +46891,7 @@ buckets {
       dimensions: "builder:linux-rel"
       dimensions: "cores:8"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Ubuntu-16.04"
+      dimensions: "os:Ubuntu-16.04|Ubuntu-18.04"
       dimensions: "pool:luci.chromium.try"
       exe {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"

infra/config/subprojects/chromium/try.star

@@ -1229,6 +1229,8 @@ try_.chromium_linux_builder(
     main_list_view = "try",
     tryjob = try_.job(),
     use_clang_coverage = True,
+    # TODO(crbug/1199425): Remove this once bionic bot is the default option.
+    os = os.LINUX_XENIAL_OR_BIONIC,
 )
 
 # Experimental builder to check dual coverage on linux platform.