Navi: Fix minor a11y issues found with audit

Bug: 882586
Change-Id: Id18e17c72aa3646a52a074e536fafa5cf789d85c
Reviewed-on: https://chromium-review.googlesource.com/c/1343246Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611818}

chrome/browser/resources/welcome/onboarding_welcome/shared/onboarding_background.html

@@ -128,16 +128,18 @@
            the blue-circle svg, because the animation needs to curve so x and y
            needs to be animated separately. -->
       <span id="blue-circle-container"></span>
-      <img id="green-rectangle"
+      <img id="green-rectangle" alt=""
           src="../images/background_svgs/green_rectangle.svg">
-      <img id="grey-oval" src="../images/background_svgs/grey_oval.svg">
-      <img id="grey-rounded-rectangle"
+      <img id="grey-oval" alt="" src="../images/background_svgs/grey_oval.svg">
+      <img id="grey-rounded-rectangle" alt=""
           src="../images/background_svgs/grey_rounded_rectangle.svg">
-      <img id="red-triangle" src="../images/background_svgs/red_triangle.svg">
-      <img id="yellow-dots" src="../images/background_svgs/yellow_dots.svg">
-      <img id="yellow-semicircle"
+      <img id="red-triangle" alt=""
+          src="../images/background_svgs/red_triangle.svg">
+      <img id="yellow-dots" alt=""
+          src="../images/background_svgs/yellow_dots.svg">
+      <img id="yellow-semicircle" alt=""
           src="../images/background_svgs/yellow_semicircle.svg">
     </div>
   </template>
   <script src="onboarding_background.js"></script>
-</dom-module>
\ No newline at end of file
+</dom-module>