fix(material/chips): chip grid not re-focusing first item

There was some logic that assumed that if a chip is the active item in the key manager, it must have focus. That's incorrect since the active item isn't reset on blur. This prevented the chip grid from re-focusing the first chip when the user tabs into it a second time.

These changes add a `focus` call whenever the grid receives focus.

Fixes angular#29785.

Author: crisbeto

src/material/chips/chip-grid.ts

@@ -336,8 +336,14 @@ export class MatChipGrid
       // Delay until the next tick, because this can cause a "changed after checked"
       // error if the input does something on focus (e.g. opens an autocomplete).
       Promise.resolve().then(() => this._chipInput.focus());
-    } else if (this._chips.length && this._keyManager.activeItemIndex !== 0) {
-      this._keyManager.setFirstItemActive();
+    } else {
+      const activeItem = this._keyManager.activeItem;
+
+      if (activeItem) {
+        activeItem.focus();
+      } else {
+        this._keyManager.setFirstItemActive();
+      }
     }
 
     this.stateChanges.next();