[Paint Preview] only capture http/https

Only capture previews of http/https pages. Some people have been
suprised when an FDT was shown on chrome://flags and capturing only
really makes sense for http/https anyways.

Change-Id: Idaaa44711abc6b2cf2cd029277f5bf23f4eae5cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377558Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802283}

chrome/browser/paint_preview/android/java/src/org/chromium/chrome/browser/paint_preview/services/PaintPreviewTabService.java

@@ -63,8 +63,10 @@ public class PaintPreviewTabService implements NativePaintPreviewServiceProvider
         }
 
         private boolean qualifiesForCapture(Tab tab) {
+            String scheme = tab.getUrl().getScheme();
+            boolean schemeAllowed = scheme.equals("http") || scheme.equals("https");
             return !tab.isIncognito() && !tab.isNativePage() && !tab.isShowingErrorPage()
-                    && tab.getWebContents() != null;
+                    && tab.getWebContents() != null && schemeAllowed;
         }
     }