Add setting to isolate zoom changes by default.

chrome/browser/resources/options/content_settings_exceptions_area.html

 <div id="content-settings-exceptions-area" class="page" hidden>
   <div class="close-button"></div>
   <h1></h1>
   <div class="content-area">
     <div id="exception-column-headers">
       <div id="exception-pattern-column" i18n-content="exceptionPatternHeader">
       </div>
       <div id="exception-behavior-column"
           i18n-content="exceptionBehaviorHeader"
           class="exception-value-column-header">
@@ skipping 127 matching lines @@
         <span class="otr-explanation" i18n-content="otrExceptionsExplanation">
         </span>
         <list mode="otr"></list>
       </div>
     </div>
     <div contentType="background-sync">
       <list mode="normal"></list>
     </div>
     <div contentType="zoomlevels">
       <list mode="normal"></list>
+      <div id="zoom-levels-ignored-explanation"

[Kevin McNee, 2017/02/03 23:17:20]

Is this a suitable way of informing the user that the zoom level exceptions are
not applied with per-tab zooming?

[wjmaclean, 2017/02/06 16:09:07]

Not sure ... we'll need a UI reviewer for that.

[Kevin McNee, 2017/02/06 18:26:11]

Sure. Shall I split up this CL first?

[wjmaclean, 2017/02/06 18:48:38]

Yeah, once you have the CL mostly how you want it, you can start thinking about
splitting it up. Though might be good to get initial buy-in from the UI reviewer
first :-)

+          i18n-content="zoomLevelsIgnoredExplanation"
+          hidden>
+      </div>
       <div>
         <span class="otr-explanation" i18n-content="otrExceptionsExplanation">
         </span>
         <list mode="otr"></list>
       </div>
     </div>
   </div>
   <div class="action-area">
     <div class="hbox stretch">
       <a target="_blank" i18n-content="learnMore"
           i18n-values="href:exceptionsLearnMoreUrl"></a>
     </div>
     <div class="action-area-right">
       <div class="button-strip">
         <button id="content-settings-exceptions-overlay-confirm"
             class="default-button" i18n-content="done">
         </button>
       </div>
     </div>
   </div>
 </div>