Add base::debug::TaskTrace::ToString() to match base::debug::StackTrace.

Change-Id: I14139ced0ed1fb23650d976784b723cf48e973ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603488
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658317}

base/debug/task_trace.cc

@@ -6,6 +6,7 @@
 
 #include <algorithm>
 #include <iostream>
+#include <sstream>
 
 #include "base/pending_task.h"
 #include "base/task/common/task_annotator.h"
@@ -52,6 +53,12 @@ void TaskTrace::OutputToStream(std::ostream* os) const {
   }
 }
 
+std::string TaskTrace::ToString() const {
+  std::stringstream stream;
+  OutputToStream(&stream);
+  return stream.str();
+}
+
 base::span<const void* const> TaskTrace::AddressesForTesting() const {
   if (empty())
     return {};

base/debug/task_trace.h

@@ -6,6 +6,7 @@
 #define BASE_DEBUG_TASK_TRACE_H_
 
 #include <iosfwd>
+#include <string>
 
 #include "base/base_export.h"
 #include "base/containers/span.h"
@@ -45,6 +46,9 @@ class BASE_EXPORT TaskTrace {
   // Outputs trace to |os|, may be called when empty() is true.
   void OutputToStream(std::ostream* os) const;
 
+  // Resolves trace to symbols and returns as string.
+  std::string ToString() const;
+
   // Returns the list of addresses in the task trace for testing.
   base::span<const void* const> AddressesForTesting() const;