Revert of Remove the last piece of deprecated synchronous IO code. (patchset #5 of...

Revert of Remove the last piece of deprecated synchronous IO code. (patchset #5 of https://codereview.chromium.org/460373002/)

Reason for revert:
Build failure on Google Chrome ChromeOS:

http://build.chromium.org/p/chromium.chrome/buildstatus?builder=Google%20Chrome%20ChromeOS&number=71164

Original issue's description:
> Remove the last piece of deprecated synchronous IO code.
>
> BUG=337096
> TEST=bots
>
> Committed: https://chromium.googlesource.com/chromium/src/+/c4a64ffdd0d511c66d774341fa0a318af7911193

TBR=dalecurtis@chromium.org,ajm@chromium.org,rkc@chromium.org,avi@chromium.org,palmer@chromium.org,xians@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=337096

Review URL: https://codereview.chromium.org/509893002

Cr-Commit-Position: refs/heads/master@{#292133}