Set kLow rather than kVeryLow priority for beacon and friends

Current the following requests have ResourceLoadPriority::kVeryLow.

 - beacon
 - a ping
 - csp report

This CL introduces a flag. When it's enabled those requests will have
ResourceLoadPriority::kLow.

Bug: 1018081
Change-Id: I68d5df324739999aaebfee6a1d7fa9c065c93013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880897Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709527}

third_party/blink/common/features.cc

@@ -404,5 +404,11 @@ const base::Feature kSubresourceRedirect{"SubresourceRedirect",
 // When 'enabled', all cross-origin iframes will get a compositing layer.
 const base::Feature kCompositeCrossOriginIframes{
     "CompositeCrossOriginIframes", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// When enabled, beacons (and friends) have ResourceLoadPriority::kLow,
+// not ResourceLoadPriority::kVeryLow.
+const base::Feature kSetLowPriorityForBeacon{"SetLowPriorityForBeacon",
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -132,6 +132,8 @@ BLINK_COMMON_EXPORT extern const base::Feature kCompositeCrossOriginIframes;
 
 BLINK_COMMON_EXPORT extern const base::Feature kSubresourceRedirect;
 
+BLINK_COMMON_EXPORT extern const base::Feature kSetLowPriorityForBeacon;
+
 }  // namespace features
 }  // namespace blink
 

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.cc

@@ -509,7 +509,11 @@ ResourceLoadPriority ResourceFetcher::ComputeLoadPriority(
                  mojom::RequestContextType::PING ||
              resource_request.GetRequestContext() ==
                  mojom::RequestContextType::CSP_REPORT) {
-    priority = ResourceLoadPriority::kVeryLow;
+    if (base::FeatureList::IsEnabled(features::kSetLowPriorityForBeacon)) {
+      priority = ResourceLoadPriority::kLow;
+    } else {
+      priority = ResourceLoadPriority::kVeryLow;
+    }
   }
 
   priority = AdjustPriorityWithPriorityHint(priority, type, resource_request,