[mb] Use symbol_level=1 for Mac Builder

This should save a lot of disk space on a Mac VM, and stop it from clobbering
the caches after every build.

Tested locally:

$ tools/mb/mb gen -m chromium.mac -b 'Mac Builder' //out/ci

Writing """\
ffmpeg_branding = "Chrome"
is_component_build = false
is_debug = false
proprietary_codecs = true
strip_absolute_paths_from_debug_symbols = true
symbol_level = 1
use_goma = true
"""

Bug: 888223
Change-Id: Ic4014f99232b1929e13dc0ca1e9b983e662a7733
Reviewed-on: https://chromium-review.googlesource.com/1249568Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594963}

tools/mb/mb_config.pyl

@@ -352,7 +352,7 @@
     },
 
     'chromium.mac': {
-      'Mac Builder': 'gpu_tests_release_bot',
+      'Mac Builder': 'gpu_tests_release_bot_minimal_symbols',
       'Mac Builder (dbg)': 'gpu_tests_debug_bot',
       'mac-jumbo-rel': 'jumbo_release_bot_minimal_symbols',
       'ios-device': 'ios',