chore(platform): fix typos in comments (angular#4415)

src/lib/core/platform/platform.ts

@@ -19,11 +19,11 @@ export class Platform {
   EDGE = /(edge)/i.test(navigator.userAgent);
   TRIDENT = /(msie|trident)/i.test(navigator.userAgent);
 
-  // EdgeHTML and Trident mock Blink specific things and need to excluded from this check.
+  // EdgeHTML and Trident mock Blink specific things and need to be excluded from this check.
   BLINK = !!(window.chrome || hasV8BreakIterator) && !!CSS && !this.EDGE && !this.TRIDENT;
 
-  // Webkit is part of the userAgent in EdgeHTML Blink and Trident, so we need to
-  // ensure that Webkit runs standalone and is not use as another engines base.
+  // Webkit is part of the userAgent in EdgeHTML, Blink and Trident. Therefore we need to
+  // ensure that Webkit runs standalone and is not used as another engine's base.
   WEBKIT = /AppleWebKit/i.test(navigator.userAgent) && !this.BLINK && !this.EDGE && !this.TRIDENT;
 
   /** Browsers and Platform Types */