viewport: convert an enum -> float constexprs

This wipes out a lot of clang warnings of the form:

third_party/blink/renderer/core/page/viewport_description.cc:125:23:
warning: comparison of floating-point type 'float' with enumeration type
'blink::ViewportDescription::(anonymous enum at
third_party/blink/renderer/core/page/viewport_description.h:71:3)'
[-Wenum-float-conversion]

Regrettably, enums can't have a `float` underlying type.

Bug: 1058218
Change-Id: I43487e8165791165337ac3df0d612aa968349ee7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087875Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747836}

third_party/blink/renderer/core/page/viewport_description.h

@@ -68,18 +68,16 @@ struct CORE_EXPORT ViewportDescription {
     kTypeCount = 7
   };
 
-  enum {
-    kValueAuto = -1,
-    kValueDeviceWidth = -2,
-    kValueDeviceHeight = -3,
-    kValuePortrait = -4,
-    kValueLandscape = -5,
-    kValueDeviceDPI = -6,
-    kValueLowDPI = -7,
-    kValueMediumDPI = -8,
-    kValueHighDPI = -9,
-    kValueExtendToZoom = -10
-  };
+  constexpr static float kValueAuto = -1.;
+  constexpr static float kValueDeviceWidth = -2.;
+  constexpr static float kValueDeviceHeight = -3.;
+  constexpr static float kValuePortrait = -4.;
+  constexpr static float kValueLandscape = -5.;
+  constexpr static float kValueDeviceDPI = -6.;
+  constexpr static float kValueLowDPI = -7.;
+  constexpr static float kValueMediumDPI = -8.;
+  constexpr static float kValueHighDPI = -9.;
+  constexpr static float kValueExtendToZoom = -10.;
 
   ViewportDescription(Type type = kUserAgentStyleSheet)
       : type(type),