[iOS][Settings] Updates IsSmallDevice

This CL defines what we consider to be a small device.

Bug: 1152730, 922511
Change-Id: I9b7fad0b45c20f823cf1655483b5c8ccff3ccfcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600902Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840614}

ios/chrome/browser/ui/util/ui_util.h

@@ -27,8 +27,6 @@ CGFloat CurrentScreenWidth();
 bool IsIPhoneX();
 
 // Returns true if the device is considered as a small device.
-// TODO(crbug.com/1152730): Define what is called a small device & update this
-// method.
 bool IsSmallDevice();
 
 // Returns the approximate corner radius of the current device.

ios/chrome/browser/ui/util/ui_util.mm

@@ -19,6 +19,15 @@
 #error "This file requires ARC support."
 #endif
 
+namespace {
+
+// The em-width value used to differentiate small and large devices.
+// With Larger Text Off, Bold Text Off and the device orientation in portrait:
+// iPhone 6s is considered as a small device, unlike iPhone 8 or iPhone 12 mini.
+const CGFloat kSmallDeviceThreshold = 22.0;
+
+}  // namespace
+
 bool IsIPadIdiom() {
   return ui::GetDeviceFormFactor() == ui::DEVICE_FORM_FACTOR_TABLET;
 }
@@ -39,7 +48,11 @@ bool IsIPhoneX() {
 }
 
 bool IsSmallDevice() {
-  return CurrentScreenWidth() <= 375;
+  CGSize mSize = [@"m" sizeWithAttributes:@{
+    NSFontAttributeName : [UIFont preferredFontForTextStyle:UIFontTextStyleBody]
+  }];
+  CGFloat emWidth = CurrentScreenWidth() / mSize.width;
+  return emWidth < kSmallDeviceThreshold;
 }
 
 CGFloat DeviceCornerRadius() {