Pre warm GPU process in WebLayer

This warms up the GPU process on startup, which has a significant impact
on perf. Pinpoint jobs show big improvements in startup with the
WebLayer shell app, but it's likely this will make even more of an
impact on client apps with larger code size, since GPU process startup
will be slower there.

Pinpoint run: https://pinpoint-dot-chromeperf.appspot.com/job/110e71ff520000
-5.1% first_contentful_paint_time
-4.3% navigation_commit_time

Bug: 1146438
Change-Id: I43e8ab3a0af14083e5a2790d8281e2e0bfa6a6ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580458
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834981}

weblayer/browser/java/org/chromium/weblayer_private/WebLayerImpl.java

@@ -155,7 +155,7 @@ public final class WebLayerImpl extends IWebLayer.Stub {
                 loadedCallbackWrapper, ValueCallback.class);
         BrowserStartupController.getInstance().startBrowserProcessesAsync(
                 LibraryProcessType.PROCESS_WEBLAYER,
-                /* startGpu */ false, /* startMinimalBrowser */ false,
+                /* startGpu */ true, /* startMinimalBrowser */ false,
                 new BrowserStartupController.StartupCallback() {
                     @Override
                     public void onSuccess() {