[Paint Preview] Accurately record shown before load ratio

Record the FirstPaintBeforeTabLoad metric to include cases where the
page has loaded before the compositor is has even started.

Change-Id: I64423a3b153779bf429ac9644d8b9c71acbaed3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401279Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805335}

chrome/browser/paint_preview/android/java/src/org/chromium/chrome/browser/paint_preview/TabbedPaintPreviewPlayer.java

@@ -66,9 +66,10 @@ public class TabbedPaintPreviewPlayer implements TabViewProvider, UserData {
 
     class TabbedPaintPreviewObserver extends EmptyTabObserver {
         public void onFirstMeaningfulPaint() {
+            mMetricsHelper.onTabLoadFinished();
+
             if (!isShowingAndNeedsBadge()) return;
 
-            mMetricsHelper.onTabLoadFinished();
             long delayMs = ChromeFeatureList.getFieldTrialParamByFeatureAsInt(
                     ChromeFeatureList.PAINT_PREVIEW_SHOW_ON_STARTUP, INITIAL_REMOVE_DELAY_PARAM,
                     DEFAULT_INITIAL_REMOVE_DELAY_MS);