[base/debug] Print task trace to Android log.

Bug: None
Test: base_unittests
Test: Call base::debug::TaskTrace().Print() somewhere on Android
Change-Id: I1a4bda16bfa11436efd277b941735d3610eaacd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666426Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672602}

base/debug/task_trace.cc

@@ -4,15 +4,50 @@
 
 #include "base/debug/task_trace.h"
 
+#include "build/build_config.h"
+
+#if defined(OS_ANDROID)
+#include <android/log.h>
+#endif  // OS_ANDROID
+
 #include <algorithm>
 #include <iostream>
 #include <sstream>
 
+#if defined(OS_ANDROID)
+#include "base/no_destructor.h"
+#endif
+
 #include "base/pending_task.h"
 #include "base/task/common/task_annotator.h"
 
 namespace base {
 namespace debug {
+namespace {
+#if defined(OS_ANDROID)
+// Android sends stdout and stderr to /dev/null; logging should be done through
+// the __android_log_write() function. Here we create an override of
+// std::stringbuf that writes to the Android log.
+class AndroidErrBuffer : public std::stringbuf {
+ protected:
+  int sync() override {
+    __android_log_write(ANDROID_LOG_ERROR, "chromium", str().c_str());
+    return 0;
+  }
+};
+
+std::ostream& DefaultOutputStream() {
+  static NoDestructor<AndroidErrBuffer> buf;
+  static NoDestructor<std::ostream> out(buf.get());
+  return *out;
+}
+#else
+// Use stderr by default.
+std::ostream& DefaultOutputStream() {
+  return std::cerr;
+}
+#endif  // OS_ANDROID
+}  // namespace
 
 TaskTrace::TaskTrace() {
   const PendingTask* current_task = TaskAnnotator::CurrentTaskForThread();
@@ -36,7 +71,7 @@ bool TaskTrace::empty() const {
 }
 
 void TaskTrace::Print() const {
-  OutputToStream(&std::cerr);
+  OutputToStream(&DefaultOutputStream());
 }
 
 void TaskTrace::OutputToStream(std::ostream* os) const {