Add DocumentTiming metrics for "time to first text paint"

Source/core/timing/PerformanceTiming.h

Index: Source/core/timing/PerformanceTiming.h
diff --git a/Source/core/timing/PerformanceTiming.h b/Source/core/timing/PerformanceTiming.h
index 08315916e71ef625a460b38b473870a7a1af1a1a..05cd018ce408ab218eae676036e611d70870ef19 100644
--- a/Source/core/timing/PerformanceTiming.h
+++ b/Source/core/timing/PerformanceTiming.h
@@ -76,7 +76,17 @@ public:
     unsigned long long domComplete() const;
     unsigned long long loadEventStart() const;
     unsigned long long loadEventEnd() const;
+
+    // The below are non-spec timings, for Page Load UMA metrics.
+
+    // The time the first document layout is performed.
     unsigned long long firstLayout() const;
+    // The time the first paint operation for visible text was performed.
+    unsigned long long firstNonBlankText() const;
+    // Similar to firstNonBlankText, but only for text with custom fonts
+    // (@font-face with url() source). This may not be recorded if no custom
+    // font is used.
+    unsigned long long firstCustomFontText() const;
 
     ScriptValue toJSONForBinding(ScriptState*) const;