[conversions] Run database sequence at TaskPriority::BEST_EFFORT

What: Re-lower conversion database sequence task priorirty.

Why: The priority was raised to USER_VISIBLE on
https://crrev.comc/2225739 in an attempt to resolve test timeouts
seen with the feature enabled. This change did not fix the flakes, and
the flakes were later addressed separately.

This returns the priority to the intended value, as conversion
operations work fine with delays.

Bug: 1080764
Change-Id: I5f5907bd5e7711b0a6ed1db45db9f3713acf4765
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340714Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796510}

content/browser/conversions/conversion_manager_impl.h

@@ -81,9 +81,6 @@ class CONTENT_EXPORT ConversionManagerImpl : public ConversionManager {
       scoped_refptr<base::SequencedTaskRunner> storage_task_runner);
 
   // |storage_task_runner| should run with base::TaskPriority::BEST_EFFORT.
-  // TODO(https://crbug.com/1080764): The storage task runner is instead run
-  // with base::TaskPriority::USER_VISIBLE to address some timeouts.
-  // Documentation should be updated.
   ConversionManagerImpl(
       StoragePartition* storage_partition,
       const base::FilePath& user_data_directory,

content/browser/storage_partition_impl.cc

@@ -1347,7 +1347,7 @@ void StoragePartitionImpl::Initialize() {
     conversion_manager_ = std::make_unique<ConversionManagerImpl>(
         this, path,
         base::ThreadPool::CreateSequencedTaskRunner(
-            {base::MayBlock(), base::TaskPriority::USER_VISIBLE}));
+            {base::MayBlock(), base::TaskPriority::BEST_EFFORT}));
   }
 
   GeneratedCodeCacheSettings settings =