media: Use USER_VISIBLE task priority for Widevine CDM component installer

When using BEST_EFFORT, the CDM is registered pretty late that browser tests
are failing, and if we specify a site playing encrypted media on command line,
the site will find Widevine not supproted (because it's not registered yet).

This CL changes the task priority to be USER_VISIBLE to solve this issue.

Bug: 900169
Test: Failing tests are passing now.
Change-Id: I8530048a07d804f5d99637745a8b7db0851ff221
Reviewed-on: https://chromium-review.googlesource.com/c/1308974
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604285}

chrome/browser/component_updater/widevine_cdm_component_installer.cc

@@ -462,8 +462,10 @@ void WidevineCdmComponentInstallerPolicy::ComponentReady(
     return;
   }
 
+  // Widevine CDM affects encrypted media playback, hence USER_VISIBLE.
+  // See http://crbug.com/900169 for the context.
   base::PostTaskWithTraits(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},
       base::BindOnce(&WidevineCdmComponentInstallerPolicy::UpdateCdmPath,
                      base::Unretained(this), version, path,
                      base::Passed(&manifest)));

components/component_updater/component_installer.cc

@@ -62,8 +62,10 @@ ComponentInstaller::~ComponentInstaller() {}
 void ComponentInstaller::Register(ComponentUpdateService* cus,
                                   base::OnceClosure callback) {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+
+  // Some components may affect user visible features, hence USER_VISIBLE.
   task_runner_ = base::CreateSequencedTaskRunnerWithTraits(
-      {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+      {base::MayBlock(), base::TaskPriority::USER_VISIBLE,
        base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN});
 
   if (!installer_policy_) {