Revert "File API: Change FileReaderLoader() CHECK to DCHECK for M76 branch."

This reverts commit c22972c6.

Reason for revert: M76 has branched. Getting the CHECK back in place so we can fix the remaining crashers.

Original change's description:
> File API: Change FileReaderLoader() CHECK to DCHECK for M76 branch.
> 
> This CL will be reverted after M76 branches, because we haven't fixed
> all the FileReaderLoader() callsites that pass in a null
> SingleThreadTaskRunner.
> 
> Bug: 957651
> Change-Id: I6a0a5bebd817853ff40c801bac3a5364e067ba3a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636504
> Auto-Submit: Victor Costan <pwnall@chromium.org>
> Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
> Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#664764}

TBR=mek@chromium.org,pwnall@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 957651
Change-Id: Ib55bff179599324618a6839b0c9a3116eba78bfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639718Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665403}

third_party/blink/renderer/core/fileapi/file_reader_loader.cc

@@ -80,7 +80,7 @@ FileReaderLoader::FileReaderLoader(
       weak_factory_(this) {
   // TODO(https://crbug.com/957651): Change this into a DCHECK once we figured
   // out where code is passing in a null task runner,
-  DCHECK(task_runner_);
+  CHECK(task_runner_);
 }
 
 FileReaderLoader::~FileReaderLoader() {