Convert enable_one_click_signin to new buildflag system This flag is only used in a few places in Chrome Review URL: https://codereview.chromium.org/1718093005 Cr-Original-Commit-Position: refs/heads/master@{#377038} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 7a5d62652384fe38454e12f9559a518924124078