Reduce the number of targets built by ToTMacASan to speed it up

The compile step currently takes around 10 hours. We get more value from
this bot cycling more quickly than we get from the extra coverage of
building all targets. Make this bot build the chromium_builder_asan
target instead of `all` to match the Windows ASan bots and the main Mac
ASan bot.

R=aeubanks@google.com, hans@chromium.org

Bug: 1151435
Change-Id: I1b954427126e36f5f83826b259d785bd84c8ff86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2555699Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834362}

testing/buildbot/chromium.clang.json

@@ -24524,7 +24524,7 @@
   },
   "ToTMacASan": {
     "additional_compile_targets": [
-      "all"
+      "chromium_builder_asan"
     ],
     "gtest_tests": [
       {

testing/buildbot/waterfalls.pyl

@@ -1612,7 +1612,7 @@
           '--test-launcher-print-test-stdio=always',
         ],
         'additional_compile_targets': [
-          'all',
+          'chromium_builder_asan',
         ],
       },
       # Used on an internal builder.