Add max-width for distilled page's settings UI and content.

This prevents the UI and the content's text from moving off the edge of
the screen at large font sizes.

Bug: 952034
Change-Id: I0d7600a79d24ea48c3fd0cc25718e74b536dd0eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838142
Auto-Submit: Aran Gilman <gilmanmh@google.com>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702583}

components/dom_distiller/core/css/distilledpage_desktop.css

@@ -27,6 +27,7 @@ fieldset {
 }
 
 #mainContent {
+  max-width: 100%;
   width: 35em;
   padding: 1em;
   margin-top: 1em;
@@ -36,6 +37,7 @@ fieldset {
 #settingsContainer {
   position: absolute;
   width: inherit;
+  max-width: 90%;
   flex: 1 1 auto;
   display: flex;
   justify-content: flex-end;