Show template before distiller finishes

This change allows content to be shown on the page prior to the
distillation of the first page of an article having completed.
The template HTML and the loading animation will show while the
distillation completes.

BUG=

Review URL: https://codereview.chromium.org/1130703003

Cr-Original-Commit-Position: refs/heads/master@{#331852}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: efe2ac1d3e1650a1cec652566542ac492eafedb3

ios/grit_whitelist.txt

@@ -328,6 +328,7 @@ IDS_DOM_DISTILLER_QUALITY_QUESTION
 IDS_DOM_DISTILLER_VIEWER_FAILED_TO_FIND_ARTICLE_CONTENT
 IDS_DOM_DISTILLER_VIEWER_FAILED_TO_FIND_ARTICLE_TITLE
 IDS_DOM_DISTILLER_VIEWER_LOADING_STRING
+IDS_DOM_DISTILLER_VIEWER_LOADING_TITLE
 IDS_DOM_DISTILLER_VIEWER_NO_DATA_CONTENT
 IDS_DOM_DISTILLER_VIEWER_NO_DATA_TITLE
 IDS_DOM_DISTILLER_VIEWER_VIEW_ORIGINAL