Update version to 6.8

TBR=machenbach@chromium.org
NOTRY=true

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ifd0eee86da0bf8b8e234b6f55ddce77239d0a3a5
Reviewed-on: https://chromium-review.googlesource.com/1009905
Commit-Queue: Michael Hablich <hablich@chromium.org>
Reviewed-by: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52570}

include/v8-version.h

@@ -9,7 +9,7 @@
 // NOTE these macros are used by some of the tool scripts and the build
 // system so their names cannot be changed without changing the scripts.
 #define V8_MAJOR_VERSION 6
-#define V8_MINOR_VERSION 7
+#define V8_MINOR_VERSION 8
 #define V8_BUILD_NUMBER 0
 #define V8_PATCH_LEVEL 0