mb: Set is_clang=false explicitly for PGO builds

The PGO builds aren't supposed to use Clang.

BUG=709690

Change-Id: I76ab708d2f187d5c6b8f59d3c46979dd27da17b7
Reviewed-on: https://chromium-review.googlesource.com/594636Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490806}

tools/mb/mb_config.pyl

@@ -1634,11 +1634,11 @@
     },
 
     'chrome_pgo_phase_1': {
-      'gn_args': 'chrome_pgo_phase=1',
+      'gn_args': 'chrome_pgo_phase=1 is_clang=false',
     },
 
     'chrome_pgo_phase_2': {
-      'gn_args': 'chrome_pgo_phase=2',
+      'gn_args': 'chrome_pgo_phase=2 is_clang=false',
     },
 
     'chrome_with_codecs': {