Add letters to SAML smart card PIN dialog keypad

As per the updated UX feedback, we're bringing the letters ("ABC",
"DEF", etc.) in the SAML smart card PIN dialog back.

The makes the UI uniform with the other contexts where the PIN
keyboard is used, and also automatically resolves the issue with the
vertical alignment of buttons in the SAML smart card dialog keypad.

Before this CL, the letters were intentionally hidden (by not
specifying the "enable-letters" attribute of <pin-keyboard>), which
was conforming to the original UX design.

Bug: 1024251
Change-Id: I12b450fd57b94f68da958b6606d9b4c85c90989e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932666Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719446}

chrome/browser/resources/chromeos/login/security_token_pin.html

@@ -26,7 +26,7 @@
       </div>
       <div slot="footer">
         <div id="pinKeyboardContainer">
-          <pin-keyboard id="pinKeyboard"
+          <pin-keyboard id="pinKeyboard" enable-letters
               has-error="[[hasError_(parameters, userEdited_)]]"
               aria-label$="[[getAriaLabel_(locale, parameters, errorLabelId_,
                                            userEdited_)]]"