Use OAuth2 FailCaller after its construction is completed

`new FailCaller` in oauth2_access_token_fetcher_immediate_error.cc may
return a stale pointer if PostTask in its ctor failed, since base::Bind
there increments the ref count, and the callback destruction releases
it.

This CL moves the PostTask after the completion of the ctor call.

Bug: 866456
Change-Id: Id185e46292238955b3da9015903dfa6a8c94e7e9
Reviewed-on: https://chromium-review.googlesource.com/1146345Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578150}

google_apis/gaia/oauth2_access_token_fetcher_immediate_error.cc

@@ -12,10 +12,6 @@
 OAuth2AccessTokenFetcherImmediateError::FailCaller::FailCaller(
     OAuth2AccessTokenFetcherImmediateError* fetcher)
     : fetcher_(fetcher) {
-  base::ThreadTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE,
-      base::Bind(&OAuth2AccessTokenFetcherImmediateError::FailCaller::run,
-                 this));
 }
 
 OAuth2AccessTokenFetcherImmediateError::FailCaller::~FailCaller() {
@@ -58,6 +54,10 @@ void OAuth2AccessTokenFetcherImmediateError::Start(
     const std::string& client_secret,
     const std::vector<std::string>& scopes) {
   failer_ = new FailCaller(this);
+  base::ThreadTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE,
+      base::BindOnce(&OAuth2AccessTokenFetcherImmediateError::FailCaller::run,
+                     failer_));
 }
 
 void OAuth2AccessTokenFetcherImmediateError::Fail() {