Change WebLayer ClassLoader parent to bootstrap ClassLoader

This is the parent the system uses when creating ClassLoaders, so it
makes sense to match that:
https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/java/android/app/ApplicationLoaders.java

Startup time improvements:
8% on android-go: https://pinpoint-dot-chromeperf.appspot.com/job/10079579520000
9% on Pixel 2: https://pinpoint-dot-chromeperf.appspot.com/job/16c9aec6520000

3.4% improvement in first contentful paint: https://pinpoint-dot-chromeperf.appspot.com/job/13b83f01520000

Bug: 1146438
Change-Id: I157fb0d69ac8e2e9a4f14887d6530d02e2748090
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521936
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825421}

weblayer/public/java/org/chromium/weblayer/WebViewCompatibilityHelper.java

@@ -46,8 +46,11 @@ final class WebViewCompatibilityHelper {
         // this to a background thread.
         StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
         try {
+            // Use the system class loader's parent here, since it is much more efficient. This
+            // matches what Android does when constructing class loaders, see
+            // android.app.ApplicationLoaders.
             return new PathClassLoader(
-                    dexPath, librarySearchPath, ClassLoader.getSystemClassLoader()) {
+                    dexPath, librarySearchPath, ClassLoader.getSystemClassLoader().getParent()) {
                 @Override
                 public Class<?> loadClass(String name) throws ClassNotFoundException {
                     // TODO(crbug.com/1112001): Investigate why loading classes causes strict mode