Update V8 to r3121 as required for WebKit update.

Change-Id: Ic53e0aef9a9eb9b71ee7d25a8aef61520bba899c
diff --git a/src/version.cc b/src/version.cc
index a36e17c..54e688b 100644
--- a/src/version.cc
+++ b/src/version.cc
@@ -34,7 +34,7 @@
 // cannot be changed without changing the SCons build script.
 #define MAJOR_VERSION     1
 #define MINOR_VERSION     3
-#define BUILD_NUMBER      14
+#define BUILD_NUMBER      17
 #define PATCH_LEVEL       0
 #define CANDIDATE_VERSION true