message_pump: Do not use contractions in #error messages

This prevents GCC from spamming the build with warnings such as

    ../../base/message_loop/message_pump_for_io.h:39:22: warning: missing terminating ' character
     #error Platform doesn't define MessagePumpForIO

Change-Id: Ice62f39141a613c2ab4a56c031826c43292f287f
Reviewed-on: https://chromium-review.googlesource.com/997632
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548424}

base/message_loop/message_pump_for_io.h

@@ -36,7 +36,7 @@ using MessagePumpForIO = MessagePumpFuchsia;
 #elif defined(OS_POSIX)
 using MessagePumpForIO = MessagePumpLibevent;
 #else
-#error Platform doesn't define MessagePumpForIO
+#error Platform does not define MessagePumpForIO
 #endif
 
 }  // namespace base

base/message_loop/message_pump_for_ui.h

@@ -47,7 +47,7 @@ using MessagePumpForUI = MessagePumpLibevent;
 #elif defined(OS_FUCHSIA)
 using MessagePumpForUI = MessagePumpFuchsia;
 #else
-#error Platform doesn't define MessagePumpForUI
+#error Platform does not define MessagePumpForUI
 #endif
 
 }  // namespace base