Fix bug where setting label images to on/off wrote over whether dialog was shown.

There was a pref name collision such that setting
kAccessibilityImageLabelsEnabled caused kAccessibilityImageLabelsOptInAccepted
to be reset. This meant that if you selected the "always" submenu option, it
would no longer remember if you'd already accepted the dialog during the
"just this once".

Bug: 905419
Change-Id: I94089366a92d76189581199c6bc1fd736b0686af
Reviewed-on: https://chromium-review.googlesource.com/c/1492043
Commit-Queue: Katie Dektar <katie@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Auto-Submit: Katie Dektar <katie@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636206}

chrome/common/pref_names.cc

@@ -1029,7 +1029,7 @@ const char kAccessibilityImageLabelsEnabled[] =
 // Whether the opt-in dialog for image labels has been accepted yet. The opt-in
 // need not be shown every time if it has already been accepted once.
 const char kAccessibilityImageLabelsOptInAccepted[] =
-    "settings.a11y.enable_accessibility_image_labels.opt_in_accepted";
+    "settings.a11y.enable_accessibility_image_labels_opt_in_accepted";
 
 #if defined(OS_MACOSX)
 // Boolean that indicates whether the application should show the info bar