Add accessible name for request pin view passphrase field

This is split out from the CL to test all widgets and views for nameless
focusable objects. See crrev.com/c/951933.

Bug: 819350
Change-Id: Ic2218d2ce520b53b6da32947d4def70a72f69c27
Reviewed-on: https://chromium-review.googlesource.com/952707Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541635}

chrome/browser/chromeos/ui/request_pin_view.cc

@@ -196,6 +196,7 @@ void RequestPinView::Init() {
   textfield_ = new PassphraseTextfield();
   textfield_->set_controller(this);
   textfield_->SetEnabled(true);
+  textfield_->SetAssociatedLabel(header_label_);
   layout->AddView(textfield_);
 
   layout->AddPaddingRow(0, related_vertical_spacing);