Enable UseParallelRequestForUnknownRangeSupport by default

Finch experiment is already 100% on stable, so enabling the feature by
default for now.
Will remove the flag if no more bugs are reported.

BUG=971366

Change-Id: I2323f995dc6e0bca68f2028be0c3014d260b5d56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004068Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732515}

components/download/public/common/download_features.cc

@@ -50,7 +50,7 @@ const base::Feature kAllowDownloadResumptionWithoutStrongValidators{
 
 const base::Feature kUseParallelRequestsForUnknwonRangeSupport{
     "UseParallelRequestForUnknownRangeSupport",
-    base::FEATURE_DISABLED_BY_DEFAULT};
+    base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kUseParallelRequestsForHTTP2{
     "UseParallelRequestsForHTTP2", base::FEATURE_DISABLED_BY_DEFAULT};