Remove unused functions WTFSetCrashHook, WTFInstallReportBacktraceOnCrashHook and WTFInvokeCrashHook.

WTFSetCrashHook, WTFInstallReportBacktraceOnCrashHook functions are not
called from anywhere, Actually these functions were only used during old WebKit
days by other WebKit ports. Now it is no longer useful for Blink, hence removing.

BUG=

Review URL: https://codereview.chromium.org/615593002

git-svn-id: svn://svn.chromium.org/blink/trunk@183667 bbb929c8-8fbe-4397-9dbb-9b2b20218538