Add kOsMac to the enable-save-password-bubble platforms.

BUG=328847

Review URL: https://codereview.chromium.org/513193002

Cr-Commit-Position: refs/heads/master@{#292601}

chrome/browser/about_flags.cc

@@ -1756,7 +1756,7 @@ const Experiment kExperiments[] = {
     "enable-save-password-bubble",
     IDS_FLAGS_ENABLE_SAVE_PASSWORD_BUBBLE_NAME,
     IDS_FLAGS_ENABLE_SAVE_PASSWORD_BUBBLE_DESCRIPTION,
-    kOsWin | kOsLinux | kOsCrOS,
+    kOsWin | kOsLinux | kOsCrOS | kOsMac,
     ENABLE_DISABLE_VALUE_TYPE(switches::kEnableSavePasswordBubble,
                               switches::kDisableSavePasswordBubble)
   },