[iOS] Add accessibility labels to text zoom buttons

Because the buttons show images, VoiceOver was reading the image
filename. There should be a real label instead.

Bug: 1028938
Change-Id: I3fdcb3e325460616d935cc594f4a7489ebbb033f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078474
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745099}

ios/chrome/app/strings/ios_strings.grd

@@ -1915,6 +1915,12 @@ Your data was encrypted with your sync passphrase on <ph name="TIME">$2<ex>Sept
       <message name="IDS_IOS_REVIEW_UPDATED_CHROME_TOS" desc="Text displayed on the New Tab Page to notify users of upcoming changes to the Terms of Serice">
         Chrome’s Terms of Service are changing March 31. <ph name="BEGIN_LINK"><ex>BEGIN_LINK</ex>BEGIN_LINK</ph>Please review<ph name="END_LINK"><ex>END_LINK</ex>END_LINK</ph>
       </message>
+      <message name="IDS_IOS_TEXT_ZOOM_ZOOM_IN" desc="Accessibility label for a button allowing the user to zoom in." meaning="User is asking page to zoom in">
+        Zoom In
+      </message>
+      <message name="IDS_IOS_TEXT_ZOOM_ZOOM_OUT" desc="Accessibility label for a button allowing the user to zoom out." meaning="User is asking page to zoom out">
+        Zoom Out
+      </message>
       <message name="IDS_IOS_TOOLBAR_CLOSE_MENU" desc="The accessibility label for the toolbar close settings menu button [iOS only].">
         Close Menu
       </message>

ios/chrome/browser/ui/text_zoom/text_zoom_view_controller.mm

@@ -136,6 +136,8 @@ const CGFloat kDividerWidth = 1;
   if (!_incrementButton) {
     _incrementButton = [self newButtonWithDefaultStyling];
     UIImage* image = [UIImage imageNamed:@"text_zoom_zoom_in"];
+    image.accessibilityLabel =
+        l10n_util::GetNSString(IDS_IOS_TEXT_ZOOM_ZOOM_IN);
     [_incrementButton setImage:image forState:UIControlStateNormal];
     [_incrementButton addTarget:self.zoomHandler
                          action:@selector(zoomIn)
@@ -154,6 +156,8 @@ const CGFloat kDividerWidth = 1;
   if (!_decrementButton) {
     _decrementButton = [self newButtonWithDefaultStyling];
     UIImage* image = [UIImage imageNamed:@"text_zoom_zoom_out"];
+    image.accessibilityLabel =
+        l10n_util::GetNSString(IDS_IOS_TEXT_ZOOM_ZOOM_OUT);
     [_decrementButton setImage:image forState:UIControlStateNormal];
     [_decrementButton addTarget:self.zoomHandler
                          action:@selector(zoomOut)