Add UMA to measure font-display value

Add WebFont.FontDisplayValue histogram that measures which fallback mode
is specified in font-display @font-face descriptor.

BUG=564447

Review URL: https://codereview.chromium.org/1647823003

Cr-Commit-Position: refs/heads/master@{#372055}

third_party/WebKit/Source/core/css/FontFace.cpp

@@ -59,6 +59,7 @@
 #include "core/frame/UseCounter.h"
 #include "platform/FontFamilyNames.h"
 #include "platform/SharedBuffer.h"
+#include "public/platform/Platform.h"
 
 namespace blink {
 
@@ -600,6 +601,9 @@ void FontFace::initCSSFontFace(Document* document, PassRefPtrWillBeRawPtr<CSSVal
         if (source)
             m_cssFontFace->addSource(source.release());
     }
+
+    if (m_display)
+        Platform::current()->histogramEnumeration("WebFont.FontDisplayValue", CSSValueToFontDisplay(m_display.get()), FontDisplayEnumMax);
 }
 
 void FontFace::initCSSFontFace(const unsigned char* data, size_t size)

third_party/WebKit/Source/core/css/RemoteFontFaceSource.h

@@ -19,7 +19,8 @@ enum FontDisplay {
     FontDisplayBlock,
     FontDisplaySwap,
     FontDisplayFallback,
-    FontDisplayOptional
+    FontDisplayOptional,
+    FontDisplayEnumMax
 };
 
 class RemoteFontFaceSource final : public CSSFontFaceSource, public FontResourceClient {

tools/metrics/histograms/histograms.xml

@@ -54748,6 +54748,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebFont.FontDisplayValue" enum="FontDisplayValue">
+  <owner>kenjibaheux@chromium.org</owner>
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    The value of font-display @font-face descriptor. Counted only when
+    explicitly specified in the @font-face rule.
+  </summary>
+</histogram>
+
 <histogram name="WebFont.HadBlankText" enum="BooleanHadBlankText">
   <owner>kenjibaheux@chromium.org</owner>
   <owner>ksakamoto@chromium.org</owner>
@@ -66752,6 +66761,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </int>
 </enum>
 
+<enum name="FontDisplayValue" type="int">
+  <int value="0" label="auto"/>
+  <int value="1" label="block"/>
+  <int value="2" label="swap"/>
+  <int value="3" label="fallback"/>
+  <int value="4" label="optional"/>
+</enum>
+
 <enum name="FormDataDeserializationStatus" type="int">
   <int value="0" label="Login database success"/>
   <int value="1" label="Login database failure"/>