Make the style statistics dump threshold configurable.

I've wanted to change the threshold a couple of times, and adding in a
10-minute opt rebuild cycle is always a bit de-motivating.

I'm making this option gecko-only because it's pretty niche, and adding
support in servo would require threading it through all the options
stuff. We can add support for it there if we ever need it.

MozReview-Commit-ID: HYlJpS3usP4

components/style/context.rs

@@ -61,10 +61,13 @@ pub struct StyleSystemOptions {
     pub disable_style_sharing_cache: bool,
     /// Whether we should dump statistics about the style system.
     pub dump_style_statistics: bool,
+    /// The minimum number of elements that must be traversed to trigger a dump
+    /// of style statistics.
+    pub style_statistics_threshold: usize,
 }
 
 #[cfg(feature = "gecko")]
-fn get_env(name: &str) -> bool {
+fn get_env_bool(name: &str) -> bool {
     use std::env;
     match env::var(name) {
         Ok(s) => !s.is_empty(),
@@ -72,6 +75,16 @@ fn get_env(name: &str) -> bool {
     }
 }
 
+const DEFAULT_STATISTICS_THRESHOLD: usize = 50;
+
+#[cfg(feature = "gecko")]
+fn get_env_usize(name: &str) -> Option<usize> {
+    use std::env;
+    env::var(name).ok().map(|s| {
+        s.parse::<usize>().expect("Couldn't parse environmental variable as usize")
+    })
+}
+
 impl Default for StyleSystemOptions {
     #[cfg(feature = "servo")]
     fn default() -> Self {
@@ -80,14 +93,17 @@ impl Default for StyleSystemOptions {
         StyleSystemOptions {
             disable_style_sharing_cache: opts::get().disable_share_style_cache,
             dump_style_statistics: opts::get().style_sharing_stats,
+            style_statistics_threshold: DEFAULT_STATISTICS_THRESHOLD,
         }
     }
 
     #[cfg(feature = "gecko")]
     fn default() -> Self {
         StyleSystemOptions {
-            disable_style_sharing_cache: get_env("DISABLE_STYLE_SHARING_CACHE"),
-            dump_style_statistics: get_env("DUMP_STYLE_STATISTICS"),
+            disable_style_sharing_cache: get_env_bool("DISABLE_STYLE_SHARING_CACHE"),
+            dump_style_statistics: get_env_bool("DUMP_STYLE_STATISTICS"),
+            style_statistics_threshold: get_env_usize("STYLE_STATISTICS_THRESHOLD")
+                                          .unwrap_or(DEFAULT_STATISTICS_THRESHOLD),
         }
     }
 }
@@ -615,6 +631,8 @@ pub struct TraversalStatistics {
     pub traversal_time_ms: f64,
     /// Whether this was a parallel traversal.
     pub is_parallel: Option<bool>,
+    /// Whether this is a "large" traversal.
+    pub is_large: Option<bool>,
 }
 
 /// Implementation of Add to aggregate statistics across different threads.
@@ -640,6 +658,7 @@ impl<'a> Add for &'a TraversalStatistics {
             stylist_rebuilds: 0,
             traversal_time_ms: 0.0,
             is_parallel: None,
+            is_large: None,
         }
     }
 }
@@ -675,7 +694,10 @@ impl TraversalStatistics {
         where E: TElement,
               D: DomTraversal<E>,
     {
+        let threshold = traversal.shared_context().options.style_statistics_threshold;
+
         self.is_parallel = Some(traversal.is_parallel());
+        self.is_large = Some(self.elements_traversed as usize >= threshold);
         self.traversal_time_ms = (time::precise_time_s() - start) * 1000.0;
         self.selectors = traversal.shared_context().stylist.num_selectors() as u32;
         self.revalidation_selectors = traversal.shared_context().stylist.num_revalidation_selectors() as u32;
@@ -688,7 +710,7 @@ impl TraversalStatistics {
     /// Returns whether this traversal is 'large' in order to avoid console spam
     /// from lots of tiny traversals.
     pub fn is_large_traversal(&self) -> bool {
-        self.elements_traversed >= 50
+        self.is_large.unwrap()
     }
 }