base: Add scope support to context trace events Make it possible to use context ids that are specific to a scope. BUG=546021 Review URL: https://codereview.chromium.org/1645853003 Cr-Original-Commit-Position: refs/heads/master@{#372967} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 8f01592f0b53ed7a1a509faf6730e9b699a27abc