Add a new file descriptor watch API.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index d3fe234e2fb6cf495078d7820c7340d012e1c330..7e60420bcd8fcbb72941a762340dafbce29b13cf 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -316,6 +316,8 @@ component("base") {
     "files/dir_reader_linux.h",
     "files/dir_reader_posix.h",
     "files/file.cc",
+    "files/file_descriptor_watcher_posix.cc",
+    "files/file_descriptor_watcher_posix.h",
     "files/file_enumerator.cc",
     "files/file_enumerator.h",
     "files/file_enumerator_posix.cc",
@@ -1123,6 +1125,8 @@ component("base") {
       "debug/crash_logging.h",
       "debug/stack_trace.cc",
       "debug/stack_trace_posix.cc",
+      "files/file_descriptor_watcher_posix.cc",
+      "files/file_descriptor_watcher_posix.h",
       "files/file_enumerator_posix.cc",
       "files/file_proxy.cc",
       "files/file_util_proxy.cc",
@@ -1781,6 +1785,7 @@ test("base_unittests") {
     "feature_list_unittest.cc",
     "file_version_info_win_unittest.cc",
     "files/dir_reader_posix_unittest.cc",
+    "files/file_descriptor_watcher_posix_unittest.cc",
     "files/file_locking_unittest.cc",
     "files/file_path_unittest.cc",
     "files/file_path_watcher_unittest.cc",