This requires a few things:

1. Make sure we set mIsLoading properly on Tab (make sure we trigger onLoadStarted, onLoadStopped, didStartPageLoad, didStopPageLoad) correctly.  We need to look into this and see if anything special needs to happen other than calling those methods.

2. We need to make sure Tab.getProgress() works properly.

Since we don't really expose the progress for now, I'd be fine with faking it and setting it to some intermediate value (30 or 50?) if we're loading, and making it 100 when we get a load complete.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8975b80a50bb91c4723c8fc29a871a940b7fdb2f

commit 8975b80a50bb91c4723c8fc29a871a940b7fdb2f
Author: shaktisahu <shaktisahu@chromium.org>
Date: Fri Oct 21 17:26:39 2016

Blimp: Propagating page loading status to tab

This CL adds support on the Blimp to propagate loading status and page
load status to Tab. This enables the UI to display the loading spinner
on the tablets and page load progress bar on phones.

The page load progress was set to 50 percent for now.

BUG= 651473 

Review-Url: https://chromiumcodereview.appspot.com/2394663002
Cr-Commit-Position: refs/heads/master@{#426827}

[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/tab/EmptyTabObserver.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/tab/TabBlimpContentsObserver.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/tab/TabObserver.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/tab/TabWebContentsObserver.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarManager.java
[modify] https://crrev.com/8975b80a50bb91c4723c8fc29a871a940b7fdb2f/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java