Begin tracking why layout is invalidated

LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt

Index: LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt
diff --git a/LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt b/LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt
index af2aef2ec10d4be28be1385c80d95444fe0db6f6..a58b1debceab75df034ee6ebe26efa1668a23016 100644
--- a/LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt
+++ b/LayoutTests/inspector/tracing/timeline-layout-with-invalidations-expected.txt
@@ -6,7 +6,7 @@ PASS
 Running: testLocalFrame
 first layout invalidations[
     {
-        cause : {reason: Layout forced, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:11}
+        cause : {reason: Style changed, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:11}
         changedAttribute : undefined
         changedClass : undefined
         changedId : undefined
@@ -19,7 +19,7 @@ first layout invalidations[
 ]
 second layout invalidations[
     {
-        cause : {reason: Layout forced, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:13}
+        cause : {reason: Style changed, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:13}
         changedAttribute : undefined
         changedClass : undefined
         changedId : undefined
@@ -34,7 +34,7 @@ second layout invalidations[
 Running: testSubframe
 first layout invalidations[
     {
-        cause : {reason: Layout forced, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:23}
+        cause : {reason: Style changed, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:23}
         changedAttribute : undefined
         changedClass : undefined
         changedId : undefined
@@ -47,7 +47,7 @@ first layout invalidations[
 ]
 second layout invalidations[
     {
-        cause : {reason: Layout forced, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:25}
+        cause : {reason: Style changed, stackTrace: .../inspector/tracing/timeline-layout-with-invalidations.html:25}
         changedAttribute : undefined
         changedClass : undefined
         changedId : undefined