Do inital read of a mojo data pipe instaed of relying on SimpleWatcher

This is cherry-picking the change made on https://crrev.com/c/1449441, expect
for enabling ResourceLoadViaDataPipe.

TBR=yhirano@chromium.org, kinuko@chromium.org

Bug: 894819, 927184, 926849
Change-Id: I229d09a68bd08722b42046c15e5a5243695eff94
Reviewed-on: https://chromium-review.googlesource.com/c/1454345Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629073}

content/renderer/loader/web_url_loader_impl.cc

@@ -920,7 +920,7 @@ void WebURLLoaderImpl::Context::OnStartLoadingResponseBody(
         MOJO_TRIGGER_CONDITION_SIGNALS_SATISFIED,
         base::BindRepeating(&WebURLLoaderImpl::Context::OnBodyAvailable, this));
     if (defers_loading_ == NOT_DEFERRING)
-      body_watcher_.ArmOrNotify();
+      OnBodyAvailable(MOJO_RESULT_OK, {});
     return;
   }