Make dynamic_annotations build with -Wconstant-conversion from newer clangs.

base/third_party/dynamic_annotations/dynamic_annotations.c

Index: base/third_party/dynamic_annotations/dynamic_annotations.c
diff --git a/base/third_party/dynamic_annotations/dynamic_annotations.c b/base/third_party/dynamic_annotations/dynamic_annotations.c
index f57cb1274502b711ac56706cac7a021ade2ab9ab..4313ecc5be7a4e2ca51414aa082d7ece0460c108 100644
--- a/base/third_party/dynamic_annotations/dynamic_annotations.c
+++ b/base/third_party/dynamic_annotations/dynamic_annotations.c
@@ -61,10 +61,10 @@
    folding them. */
 #ifdef __COUNTER__
 #define DYNAMIC_ANNOTATIONS_IMPL \
-  volatile short lineno = (__LINE__ << 8) + __COUNTER__; (void)lineno;
+  volatile unsigned short lineno = (__LINE__ << 8) + __COUNTER__; (void)lineno;
 #else
 #define DYNAMIC_ANNOTATIONS_IMPL \
-  volatile short lineno = (__LINE__ << 8); (void)lineno;
+  volatile unsigned short lineno = (__LINE__ << 8); (void)lineno;
 #endif
 
 /* WARNING: always add new annotations to the end of the list.