Mac: Opt out of Mojave dark mode

This is temporary until we have bandwidth to do a correct dark mode
implementation. Opting out fixes glitches caused by the system thinking
that Chrome is in dark mode (for example, light text colors where we
are expecting the system to provide dark text colors).

Bug: 850103
Change-Id: I38d46631f714907231da89bd47bdd800ceb1bb3e
Reviewed-on: https://chromium-review.googlesource.com/1120744Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571561}

chrome/app/app-Info.plist

@@ -350,5 +350,7 @@
 	</array>
         <key>NSUserNotificationAlertStyle</key>
         <string>banner</string>
+        <key>NSRequiresAquaSystemAppearance</key>
+        <true/>
 </dict>
 </plist>