Reshuffle include/ directory layout

This CL reorganizes the include/ directory as follows:
1. The immediate subdirectories of include/ are now the stable API
   surface exposed to all embedders, with the exception of ext/.
2. ext/ contains the unstable API surface that is exposed only to chrome.

Test: builds on all configurations and passes tests
Bug: 132678367
Change-Id: If638d4b4c150fd591cd1db851283b25ff6083ed6

src/base/thread_task_runner_unittest.cc

diff --git a/src/base/thread_task_runner_unittest.cc b/src/base/thread_task_runner_unittest.cc
index 0eb3a76..c4356fe 100644
--- a/src/base/thread_task_runner_unittest.cc
+++ b/src/base/thread_task_runner_unittest.cc
@@ -14,12 +14,12 @@
  * limitations under the License.
  */
 
-#include "perfetto/base/thread_task_runner.h"
+#include "perfetto/ext/base/thread_task_runner.h"
 
 #include <thread>
 
 #include <gtest/gtest.h>
-#include "perfetto/base/thread_checker.h"
+#include "perfetto/ext/base/thread_checker.h"
 
 namespace perfetto {
 namespace base {