predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966,655980

Review-Url: https://codereview.chromium.org/2589713002
Cr-Commit-Position: refs/heads/master@{#439453}

chrome/browser/predictors/resource_prefetch_common.cc

@@ -134,7 +134,7 @@ ResourcePrefetchPredictorConfig::ResourcePrefetchPredictorConfig()
       max_consecutive_misses(3),
       min_resource_confidence_to_trigger_prefetch(0.7f),
       min_resource_hits_to_trigger_prefetch(2),
-      max_prefetches_inflight_per_navigation(24),
+      max_prefetches_inflight_per_navigation(5),
       max_prefetches_inflight_per_host_per_navigation(3) {
 }
 

chrome/browser/predictors/resource_prefetcher_unittest.cc

@@ -144,7 +144,6 @@ ResourcePrefetcherTest::ResourcePrefetcherTest()
     : loop_(base::MessageLoop::TYPE_IO),
       io_thread_(content::BrowserThread::IO, &loop_),
       prefetcher_delegate_(&loop_) {
-  config_.max_prefetches_inflight_per_navigation = 5;
   config_.max_prefetches_inflight_per_host_per_navigation = 2;
 }