MD Settings: Keep sliding subpages within card

The sliding subpages in neon-animated-pages (settings-animated-pages)
shouldn't protrude past the edges of the card when sliding left/right. A
stacking context ensures this without screwing up positioning of the card.

BUG=640315
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2264353004
Cr-Commit-Position: refs/heads/master@{#414866}

chrome/browser/resources/settings/settings_page/settings_section.html

@@ -36,6 +36,8 @@
       :host(.collapsing) #card,
       :host(.expanded) #card {
         @apply(--shadow-elevation-4dp);
+        /* A stacking context constrains sliding sub-pages to the card. */
+        z-index: 0;
       }
 
       :host(:not(.expanding):not(.expanded)) #card {