Update WebKit to preview-pr-135-a6fa914b

This updates the WebKit version to use the preview build from PR #135
which fixes async context preservation across await for AsyncLocalStorage.

Fixes the failing test: test-diagnostics-channel-tracing-channel-promise-run-stores.js

cmake/tools/SetupWebKit.cmake

@@ -2,7 +2,7 @@ option(WEBKIT_VERSION "The version of WebKit to use")
 option(WEBKIT_LOCAL "If a local version of WebKit should be used instead of downloading")
 
 if(NOT WEBKIT_VERSION)
-  set(WEBKIT_VERSION d5bd162d9ab2ce0b408f0f1aa499e550add33880)
+  set(WEBKIT_VERSION preview-pr-135-a6fa914b)
 endif()
 
 string(SUBSTRING ${WEBKIT_VERSION} 0 16 WEBKIT_VERSION_PREFIX)