Add an "append flag" to base::PlatformFile.

base/platform_file_posix.cc

Index: base/platform_file_posix.cc
diff --git a/base/platform_file_posix.cc b/base/platform_file_posix.cc
index beae804c629c86a6b8fa4ef716f601552d9a114e..a8be8a4a95ec4351c824f1fece257a00af033dfc 100644
--- a/base/platform_file_posix.cc
+++ b/base/platform_file_posix.cc
@@ -81,6 +81,7 @@ PlatformFile CreatePlatformFileUnsafe(const FilePath& name,
     open_flags |= O_WRONLY;
   } else if (!(flags & PLATFORM_FILE_READ) &&
              !(flags & PLATFORM_FILE_WRITE_ATTRIBUTES) &&
+             !(flags & PLATFORM_FILE_APPEND) &&
              !(flags & PLATFORM_FILE_OPEN_ALWAYS)) {
     NOTREACHED();
   }
@@ -88,6 +89,11 @@ PlatformFile CreatePlatformFileUnsafe(const FilePath& name,
   if (flags & PLATFORM_FILE_TERMINAL_DEVICE)
     open_flags |= O_NOCTTY | O_NDELAY;
 
+  if (flags & PLATFORM_FILE_APPEND && flags & PLATFORM_FILE_READ)
+    open_flags |= O_APPEND | O_RDWR;
+  else if (flags & PLATFORM_FILE_APPEND)
+    open_flags |= O_APPEND | O_WRONLY;
+
   COMPILE_ASSERT(O_RDONLY == 0, O_RDONLY_must_equal_zero);
 
   int mode = S_IRUSR | S_IWUSR;