[blink] Add flag for small script streaming

Add a flag to enable streaming small (<30kB) scripts. With the improved
streaming capabilities and reduced background parsing costs, it may be
beneficial to stream all scripts, not just the large ones.

Bug: 865098
Change-Id: I211bbbc8491050041ebcbb4e9d6019263fcfd4eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632430
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664205}

third_party/blink/common/features.cc

@@ -36,6 +36,10 @@ const base::Feature kEnableGpuRasterizationViewportRestriction{
 const base::Feature kScriptStreaming{"ScriptStreaming",
                                      base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Allow streaming small (<30kB) scripts.
+const base::Feature kSmallScriptStreaming{"SmallScriptStreaming",
+                                          base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Enables user level memory pressure signal generation on Android.
 const base::Feature kUserLevelMemoryPressureSignal{
     "UserLevelMemoryPressureSignal", base::FEATURE_DISABLED_BY_DEFAULT};

third_party/blink/public/common/features.h

@@ -19,6 +19,7 @@ BLINK_COMMON_EXPORT extern const base::Feature
 BLINK_COMMON_EXPORT extern const base::Feature
     kEnableGpuRasterizationViewportRestriction;
 BLINK_COMMON_EXPORT extern const base::Feature kScriptStreaming;
+BLINK_COMMON_EXPORT extern const base::Feature kSmallScriptStreaming;
 BLINK_COMMON_EXPORT extern const base::Feature kUserLevelMemoryPressureSignal;
 BLINK_COMMON_EXPORT extern const base::Feature kFirstContentfulPaintPlusPlus;
 BLINK_COMMON_EXPORT extern const base::Feature kFreezePurgeMemoryAllPagesFrozen;

third_party/blink/renderer/bindings/core/v8/script_streamer.cc

@@ -11,6 +11,7 @@
 #include "base/threading/scoped_blocking_call.h"
 #include "base/threading/thread_restrictions.h"
 #include "mojo/public/cpp/system/wait.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/bindings/core/v8/v8_code_cache.h"
 #include "third_party/blink/renderer/core/dom/document.h"
 #include "third_party/blink/renderer/core/dom/element.h"
@@ -24,7 +25,6 @@
 #include "third_party/blink/renderer/platform/loader/fetch/cached_metadata.h"
 #include "third_party/blink/renderer/platform/loader/fetch/resource.h"
 #include "third_party/blink/renderer/platform/loader/fetch/response_body_loader.h"
-#include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 #include "third_party/blink/renderer/platform/scheduler/public/post_cross_thread_task.h"
 #include "third_party/blink/renderer/platform/scheduler/public/thread_scheduler.h"
 #include "third_party/blink/renderer/platform/scheduler/public/worker_pool.h"
@@ -354,8 +354,12 @@ void RunScriptStreamingTask(
 }  // namespace
 
 bool ScriptStreamer::HasEnoughDataForStreaming(size_t resource_buffer_size) {
-  // Only stream larger scripts.
-  return resource_buffer_size >= small_script_threshold_;
+  if (base::FeatureList::IsEnabled(features::kSmallScriptStreaming)) {
+    return resource_buffer_size >= kMaximumLengthOfBOM;
+  } else {
+    // Only stream larger scripts.
+    return resource_buffer_size >= small_script_threshold_;
+  }
 }
 
 // Try to start streaming the script from the given datapipe, taking ownership