Do not show the previews icon on broken HTTPS pages

This was in the original UX spec and I missed it :(
It caused the bug below since the VerboseStatusView does respect the
security level, but this did not.

On a side note, I've filed a bug for M74 to not allow previews on
broken HTTPS pages in the first place.

Bug: 929412
Change-Id: If9ad72584de62e6a51e4a6e676255d5e394ba9c3
Reviewed-on: https://chromium-review.googlesource.com/c/1474382Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632653}

chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java

@@ -936,6 +936,7 @@ public class Tab
      */
     public boolean isPreview() {
         return getWebContents() != null && !isNativePage() && !isShowingInterstitialPage()
+                && getSecurityLevel() != ConnectionSecurityLevel.DANGEROUS
                 && PreviewsAndroidBridge.getInstance().shouldShowPreviewUI(getWebContents());
     }