Prevent accessing mUseLightColorAssets being null

mUseLightColorAssets could be null, which can cause IPH crashes on
tablets if called before it is set.  This just protects against the
value not being ready.

BUG=778033

Change-Id: I057178d966d44ee1d755241be919d48e1e324e37
Reviewed-on: https://chromium-review.googlesource.com/779864
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519736}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarTablet.java

@@ -552,14 +552,14 @@ public class ToolbarTablet
 
     @Override
     public boolean useLightDrawables() {
-        return mUseLightColorAssets;
+        return mUseLightColorAssets != null && mUseLightColorAssets;
     }
 
     @Override
     public void showAppMenuUpdateBadge() {
         super.showAppMenuUpdateBadge();
         if (!mIsInTabSwitcherMode) {
-            if (mUseLightColorAssets) {
+            if (mUseLightColorAssets != null && mUseLightColorAssets) {
                 setAppMenuUpdateBadgeDrawable(mUseLightColorAssets);
             }
             setAppMenuUpdateBadgeToVisible(true);