webp/types.h: use inline for clang++/-std=c++11

at least clang 3.[45] in c++ mode with -std=c++11 define __STRICT_ANSI__
this change set WEBP_INLINE to inline for c++/non-strict-ansi/> c99

fixes crbug.com/428383

Change-Id: Ief2b934353c336a75865c73c90cc3dc5e4f83913
Merge "webp/types.h: use inline for clang++/-std=c++11"