Make the QR Code scanner caption fit on multiple lines.

ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm

Index: ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm
diff --git a/ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm b/ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm
index 2101964d24e869103647dbd2bad3b2520181af46..d9fce117737114df4be2f6df24a79b8fd09c529c 100644
--- a/ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm
+++ b/ios/chrome/browser/ui/qr_scanner/qr_scanner_view.mm
@@ -41,7 +41,9 @@ const CGFloat kViewportBorderShadowOpacity = 1.0;
 // Shadow radius of the viewport border.
 const CGFloat kViewportBorderShadowRadius = 10.0;
 // Padding of the viewport caption, below the viewport.
-const CGFloat kViewportCaptionPadding = 24.0;
+const CGFloat kViewportCaptionVerticalPadding = 14.0;
+// Padding of the viewport caption from the edges of the superview.
+const CGFloat kViewportCaptionHorizontalPadding = 20.0;
 // Shadow opacity of the viewport caption.
 const CGFloat kViewportCaptionShadowOpacity = 1.0;
 // Shadow radius of the viewport caption.
@@ -470,6 +472,8 @@ CGFloat GetViewportSize() {
   UILabel* viewportCaption = [[[UILabel alloc] init] autorelease];
   NSString* label = l10n_util::GetNSString(IDS_IOS_QR_SCANNER_VIEWPORT_CAPTION);
   [viewportCaption setText:label];
+  [viewportCaption setNumberOfLines:0];
+  [viewportCaption setTextAlignment:NSTextAlignmentCenter];
   [viewportCaption setAccessibilityLabel:label];
   [viewportCaption setAccessibilityIdentifier:@"qr_scanner_viewport_caption"];
   [viewportCaption setTextColor:[UIColor whiteColor]];
@@ -486,9 +490,15 @@ CGFloat GetViewportSize() {
   [NSLayoutConstraint activateConstraints:@[
     [[viewportCaption centerXAnchor]
         constraintEqualToAnchor:[self centerXAnchor]],
-    [[viewportCaption centerYAnchor]
+    [[viewportCaption topAnchor]

[gambard, 2017/02/22 10:49:04]

Don't we want to just align on the _previewView.bottomAnchor? Or is there a
scenario where the preview is not shown and we want the text to be displayed
here?

[jif, 2017/02/22 12:42:15]

_previewView is the view that contains the video. It takes the whole screen.
What we'd need is the bottomAnchor of the transparent square, but that
transparent square is not a view, so we can't use its bottomAnchor.

         constraintEqualToAnchor:[self centerYAnchor]
-                       constant:GetViewportSize() / 2 + kViewportCaptionPadding]
+                       constant:GetViewportSize() / 2 +
+                                kViewportCaptionVerticalPadding],
+    [viewportCaption.widthAnchor

[gambard, 2017/02/22 10:49:04]

Why using this and not constraints on leading/trailing + constants?
The result would be the same but it would be easier to read in my opinion.

[jif, 2017/02/22 12:42:15]

Done.

+        constraintLessThanOrEqualToAnchor:self.widthAnchor
+                               multiplier:1.0
+                                 constant:-2 *
+                                          kViewportCaptionHorizontalPadding]
   ]];
 }