Control the HTML parser yield interval via field trial.

The parser currently yields every 500ms unless it encounters a script tag. We will experiment
with different values and observe the impact.

https://goo.gl/zwQWyW

Change-Id: I1dcec74fb41e438393e505323707c00d76b49d15
Reviewed-on: https://chromium-review.googlesource.com/1148172
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582554}

third_party/blink/renderer/core/html/parser/html_parser_scheduler.cc

@@ -25,6 +25,8 @@
 
 #include "third_party/blink/renderer/core/html/parser/html_parser_scheduler.h"
 
+#include "base/feature_list.h"
+#include "base/metrics/field_trial_params.h"
 #include "third_party/blink/public/platform/platform.h"
 #include "third_party/blink/public/platform/web_thread.h"
 #include "third_party/blink/renderer/core/dom/document.h"
@@ -35,6 +37,10 @@
 
 namespace blink {
 
+const base::Feature kHTMLParsingYieldTime {
+  "HTMLParsingYieldTime", base::FEATURE_DISABLED_BY_DEFAULT
+};
+
 PumpSession::PumpSession(unsigned& nesting_level)
     : NestingLevelIncrementer(nesting_level) {}
 
@@ -55,12 +61,18 @@ void SpeculationsPumpSession::AddedElementTokens(size_t count) {
   processed_element_tokens_ += count;
 }
 
+const double kDefaultParserTimeLimit = 0.5;
+
 HTMLParserScheduler::HTMLParserScheduler(
     HTMLDocumentParser* parser,
     scoped_refptr<base::SingleThreadTaskRunner> loading_task_runner)
     : parser_(parser),
       loading_task_runner_(std::move(loading_task_runner)),
-      is_paused_with_active_timer_(false) {}
+      is_paused_with_active_timer_(false),
+      parser_time_limit_(
+          base::GetFieldTrialParamByFeatureAsDouble(kHTMLParsingYieldTime,
+                                                    "limit",
+                                                    kDefaultParserTimeLimit)) {}
 
 HTMLParserScheduler::~HTMLParserScheduler() = default;
 
@@ -111,8 +123,7 @@ inline bool HTMLParserScheduler::ShouldYield(
           ->ShouldYieldForHighPriorityWork())
     return true;
 
-  const double kParserTimeLimit = 0.5;
-  if (session.ElapsedTime() > kParserTimeLimit)
+  if (session.ElapsedTime() > parser_time_limit_)
     return true;
 
   // Yield if a lot of DOM work has been done in this session and a script tag

third_party/blink/renderer/core/html/parser/html_parser_scheduler.h

@@ -105,6 +105,7 @@ class HTMLParserScheduler final
 
   TaskHandle cancellable_continue_parse_task_handle_;
   bool is_paused_with_active_timer_;
+  const double parser_time_limit_;
 
   DISALLOW_COPY_AND_ASSIGN(HTMLParserScheduler);
 };