Add main thread scrolling reasons to GraphicsLayer::layerTreeAsJSON.

cc/input/main_thread_scrolling_reason.h

Index: cc/input/main_thread_scrolling_reason.h
diff --git a/cc/input/main_thread_scrolling_reason.h b/cc/input/main_thread_scrolling_reason.h
index 522909afb1eaed1b959ee19f6abce3a53d13f053..9ae15b3b7768677e876e72527569a7b6b9068f26 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -5,6 +5,10 @@
 #ifndef CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 #define CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 
+#include <sstream>
+#include <string>
+#include <vector>
+
 namespace cc {
 
 // Ensure this stays in sync with MainThreadScrollingReason in histograms.xml.
@@ -55,6 +59,66 @@ struct MainThreadScrollingReason {
         kNonInvertibleTransform | kPageBasedScrolling;
     return (reasons & reasons_set_by_compositor) == reasons;
   }
+
+  static std::string mainThreadScrollingReasonsAsText(uint32_t reasons) {
+    std::stringstream ss;
+    std::vector<std::string> result_as_array =
+        mainThreadScrollingReasonsAsStringArray(reasons);
+    size_t array_length = result_as_array.size();
+
+    for (size_t i = 0; i < array_length - 1; ++i) {
+      ss << result_as_array[i] << ", ";
+    }
+    if (array_length) {
+      ss << result_as_array[array_length - 1];
+      return ss.str();
+    }
+
+    return "";
+  }
+
+  static std::vector<std::string> mainThreadScrollingReasonsAsStringArray(
+      uint32_t reasons) {

[Ian Vollick, 2016/07/12 15:12:13]

Perhaps this could convert to a TracedValue directly, and the
mainThreadScrollingReasonsAsText could just call ToString on it?

[sahel, 2016/07/12 17:20:59]

The format of the text that stringCoordinator needs is neither a key-value pairs
nor array. It is a single string with comma separated names. Should I change the
ScrollingCoordinator::mainThreadScrollingReasonsAsText() function and where it's
called as well?

[sahel, 2016/07/12 20:35:24]

Done.

+    std::vector<std::string> result;
+    if (reasons &
+        MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects)
+      result.push_back("Has background-attachment:fixed");
+    if (reasons &
+        MainThreadScrollingReason::kHasNonLayerViewportConstrainedObjects)
+      result.push_back("Has non-layer viewport-constrained objects");
+    if (reasons & MainThreadScrollingReason::kThreadedScrollingDisabled)
+      result.push_back("Threaded scrolling is disabled");
+    if (reasons & MainThreadScrollingReason::kScrollbarScrolling)
+      result.push_back("Scrollbar scrolling");
+    if (reasons & MainThreadScrollingReason::kPageOverlay)
+      result.push_back("Page overlay");
+    if (reasons & MainThreadScrollingReason::kAnimatingScrollOnMainThread)
+      result.push_back("Animating scroll on main thread");
+    if (reasons & MainThreadScrollingReason::kHasStickyPositionObjects)
+      result.push_back("Has sticky position objects");
+    if (reasons & MainThreadScrollingReason::kCustomScrollbarScrolling)
+      result.push_back("Custom scrollbar scrolling");
+
+    // Transient scrolling reasons.
+    if (reasons & MainThreadScrollingReason::kNonFastScrollableRegion)
+      result.push_back("Non fast scrollable region");
+    if (reasons & MainThreadScrollingReason::kEventHandlers)
+      result.push_back("Event handlers");
+    if (reasons & MainThreadScrollingReason::kFailedHitTest)
+      result.push_back("Failed hit test");
+    if (reasons & MainThreadScrollingReason::kNoScrollingLayer)
+      result.push_back("No scrolling layer");
+    if (reasons & MainThreadScrollingReason::kNotScrollable)
+      result.push_back("Not scrollable");
+    if (reasons & MainThreadScrollingReason::kContinuingMainThreadScroll)
+      result.push_back("Continuing main thread scroll");
+    if (reasons & MainThreadScrollingReason::kNonInvertibleTransform)
+      result.push_back("Non-invertible transform");
+    if (reasons & MainThreadScrollingReason::kPageBasedScrolling)
+      result.push_back("Page-based scrolling");
+
+    return result;
+  }
 };
 
 }  // namespace cc