Type this command to run Safari with the WebKit you built:
WebKit/WebKitTools/Scripts/run-safari
Use the --debug option for a debug build:
WebKit/WebKitTools/Scripts/run-safari --debug
The run-safari script sets the DYLD_FRAMEWORK_PATH environment variable to point to your build products, and then launches /Applications/Safari.app. DYLD_FRAMEWORK_PATH tells the system loader to prefer your build products over the frameworks installed in /System/Library/Frameworks.
It's handy to put WebKit/WebKitTools/Scripts in your shell path so you can type commands like run-safari without specifying a full path.