[DevTools] Do not print pipe I/O errors when shutting down

When browser shutdown started pipe file descriptors are closed
on the UI thread while there may be outstanding reads and writes
on parallel threads. If such read/write operations fail error
messages will only be written to the console if pipe shutdown
has not begun yet.

Bug: none
Change-Id: I57e9b6c7f2921335cc2bb1f5289bd9347bba1f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108825Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Yury Semikhatsky <yurys@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751605}

content/browser/devtools/devtools_pipe_handler.h

@@ -10,13 +10,10 @@
 #include "base/memory/weak_ptr.h"
 #include "content/public/browser/devtools_agent_host_client.h"
 
-namespace base {
-class Thread;
-}
-
 namespace content {
 
 class PipeReaderBase;
+class PipeWriterBase;
 
 class DevToolsPipeHandler : public DevToolsAgentHostClient {
  public:
@@ -45,8 +42,7 @@ class DevToolsPipeHandler : public DevToolsAgentHostClient {
   ProtocolMode mode_;
 
   std::unique_ptr<PipeReaderBase> pipe_reader_;
-  std::unique_ptr<base::Thread> read_thread_;
-  std::unique_ptr<base::Thread> write_thread_;
+  std::unique_ptr<PipeWriterBase> pipe_writer_;
   scoped_refptr<DevToolsAgentHost> browser_target_;
   int read_fd_;
   int write_fd_;