Update QR Code scanner caption string and padding.

ios/chrome/app/strings/ios_strings.grd

Index: ios/chrome/app/strings/ios_strings.grd
diff --git a/ios/chrome/app/strings/ios_strings.grd b/ios/chrome/app/strings/ios_strings.grd
index 88bdcef1e484605d119d1e5295e32410840360cf..d2343cdf5763136f2b0f8e4e7a579070c6dcc9a6 100644
--- a/ios/chrome/app/strings/ios_strings.grd
+++ b/ios/chrome/app/strings/ios_strings.grd
@@ -1051,7 +1051,7 @@ Handoff must also be enabled in the General section of Settings, and your device
         On
       </message>
       <message name="IDS_IOS_QR_SCANNER_VIEWPORT_CAPTION" desc="Message displayed below the QR scanner viewport instructing the user to position the scanned QR or bar code inside the viewport. [Length: unlimited] [iOS only]">
-        QR Code or barcode
+        Position QR code or barcode in this frame
       </message>
       <message name="IDS_IOS_READING_LIST_SNACKBAR_MESSAGE" desc="Message briefly displayed at the bottom of the screen to the user to inform that the selected page has been added to the reading list. [Length: 35em]" meaning="The selected page has been added to your reading list. [Length: 35em]">
         Added to Reading List