Rewrite getClassFromElement() in a functional style.

This is part of a series of changes re-implementing (and hopefully
improving upon) cleanup performed in https://crrev.com/c/1922788. The
intent is to reduce the number of changes not directly related to the
new feature when relanding the CL.

Bug: 1027612

Change-Id: Ibbc6967e43695f348f53655e46d05a547f8bd2ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144674Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Auto-Submit: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#758997}

components/dom_distiller/core/javascript/dom_distiller_viewer.js

@@ -80,14 +80,10 @@ function useTheme(theme) {
   updateToolbarColor(theme);
 }
 
-function getClassFromElement(element, classList) {
-  let foundClass = classList[0];
-  classList.forEach((cls) => {
-    if (element.classList.contains(cls)) {
-      foundClass = cls;
-    }
-  });
-  return foundClass;
+function getPageTheme() {
+  const cls = Array.from(document.body.classList)
+                  .find((cls) => themeClasses.includes(cls));
+  return cls ? cls : themeClasses[0];
 }
 
 function updateToolbarColor(theme) {
@@ -166,7 +162,9 @@ class FontSizeSlider {
 const fontSizeSlider = new FontSizeSlider(
     $('font-size-selection'), [14, 15, 16, 18, 20, 24, 28, 32, 40, 48]);
 
-updateToolbarColor(getClassFromElement(document.body, themeClasses));
+// Set the toolbar color to match the page's theme.
+updateToolbarColor(getPageTheme());
+
 maybeSetWebFont();
 
 // The zooming speed relative to pinching speed.