[threadpool] Set nacl tasks required for official keyboard on ChromeOS as USER_BLOCKING.

This is a required step to make the official keyboard work without
BEST_EFFORT tasks.

BEST_EFFORT tasks can be arbitrarily delayed. They should not be on
the critical path of behavior the user is actively waiting on.

Test:
- On a branch that has both this CL and
  https://chromium-review.googlesource.com/c/chromium/src/+/1680760,
  run Chrome with --enable-virtual-keyboard --disable-best-effort-tasks.
- Click on the omnibox to enter text.
- Expected result: The official virtual keyboard is displayed.

Bug: 976542
Change-Id: I05338afd019d769da18c272dbf300128b8d891bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693184Reviewed-by: Derek Schuff <dschuff@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675825}

components/nacl/browser/nacl_browser.cc

@@ -31,6 +31,10 @@
 
 namespace {
 
+// Tasks posted in this file are on the critical path of displaying the official
+// virtual keyboard on Chrome OS. https://crbug.com/976542
+constexpr base::TaskPriority kUserBlocking = base::TaskPriority::USER_BLOCKING;
+
 // An arbitrary delay to coalesce multiple writes to the cache.
 const int kValidationCacheCoalescingTimeMS = 6000;
 const base::FilePath::CharType kValidationCacheFileName[] =
@@ -281,7 +285,7 @@ void NaClBrowser::EnsureIrtAvailable() {
   if (IsOk() && irt_state_ == NaClResourceUninitialized) {
     irt_state_ = NaClResourceRequested;
     auto task_runner = base::CreateTaskRunnerWithTraits(
-        {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+        {base::MayBlock(), kUserBlocking,
          base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN});
     std::unique_ptr<base::FileProxy> file_proxy(
         new base::FileProxy(task_runner.get()));
@@ -375,7 +379,7 @@ void NaClBrowser::EnsureValidationCacheAvailable() {
       // task and further file access will not occur until after we get a
       // response.
       base::PostTaskWithTraitsAndReply(
-          FROM_HERE, {base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+          FROM_HERE, {base::MayBlock(), kUserBlocking},
           base::BindOnce(ReadCache, validation_cache_file_path_, data),
           base::BindOnce(&NaClBrowser::OnValidationCacheLoaded,
                          base::Unretained(this), base::Owned(data)));

components/nacl/browser/nacl_process_host.cc

@@ -811,7 +811,10 @@ bool NaClProcessHost::StartNaClExecution() {
       // compromised renderer to pass an arbitrary fd that could get loaded
       // into the plugin process.
       base::PostTaskWithTraitsAndReplyWithResult(
-          FROM_HERE, {base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+          FROM_HERE,
+          // USER_BLOCKING because it is on the critical path of displaying the
+          // official virtual keyboard on Chrome OS. https://crbug.com/976542
+          {base::MayBlock(), base::TaskPriority::USER_BLOCKING},
           base::BindOnce(OpenNaClReadExecImpl, file_path,
                          true /* is_executable */),
           base::BindOnce(&NaClProcessHost::StartNaClFileResolved,
@@ -1044,7 +1047,10 @@ void NaClProcessHost::OnResolveFileToken(uint64_t file_token_lo,
 
   // Open the file.
   base::PostTaskWithTraitsAndReplyWithResult(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+      FROM_HERE,
+      // USER_BLOCKING because it is on the critical path of displaying the
+      // official virtual keyboard on Chrome OS. https://crbug.com/976542
+      {base::MayBlock(), base::TaskPriority::USER_BLOCKING},
       base::Bind(OpenNaClReadExecImpl, file_path, true /* is_executable */),
       base::Bind(&NaClProcessHost::FileResolved, weak_factory_.GetWeakPtr(),
                  file_token_lo, file_token_hi, file_path));