fix(tabs): use buttons for paginator

Uses `button` elements, rather than styled `div`-s for the paginator buttons. This has the advantage of being more accessible, if we decided to make them focusable, and it stops calling the click listeners when the button is disabled (currently we do some expensive checks in them when we don't have to).

Author: crisbeto

src/lib/tabs/tab-header.html

@@ -1,10 +1,14 @@
-<div class="mat-tab-header-pagination mat-tab-header-pagination-before mat-elevation-z4"
+<button class="mat-tab-header-pagination mat-tab-header-pagination-before mat-elevation-z4"
      aria-hidden="true"
-     mat-ripple [matRippleDisabled]="_disableScrollBefore || disableRipple"
+     type="button"
+     mat-ripple
+     [matRippleDisabled]="_disableScrollBefore || disableRipple"
      [class.mat-tab-header-pagination-disabled]="_disableScrollBefore"
+     [disabled]="_disableScrollBefore || null"
+     tabindex="-1"
      (click)="_scrollHeader('before')">
   <div class="mat-tab-header-pagination-chevron"></div>
-</div>
+</button>
 
 <div class="mat-tab-label-container" #tabListContainer
      (keydown)="_handleKeydown($event)">
@@ -16,10 +20,14 @@
   </div>
 </div>
 
-<div class="mat-tab-header-pagination mat-tab-header-pagination-after mat-elevation-z4"
+<button class="mat-tab-header-pagination mat-tab-header-pagination-after mat-elevation-z4"
      aria-hidden="true"
-     mat-ripple [matRippleDisabled]="_disableScrollAfter || disableRipple"
+     type="button"
+     mat-ripple
+     [matRippleDisabled]="_disableScrollAfter || disableRipple"
      [class.mat-tab-header-pagination-disabled]="_disableScrollAfter"
+     [disabled]="_disableScrollAfter || null"
+     tabindex="-1"
      (click)="_scrollHeader('after')">
   <div class="mat-tab-header-pagination-chevron"></div>
-</div>
+</button>

src/lib/tabs/tab-header.scss

@@ -32,6 +32,15 @@
   min-width: 32px;
   cursor: pointer;
   z-index: 2;
+  box-sizing: content-box;
+  background: none;
+  border: none;
+  outline: 0;
+  padding: 0;
+
+  &::-moz-focus-inner {
+    border: 0;
+  }
 
   .mat-tab-header-pagination-controls-enabled & {
     display: flex;