Add more string_util functions to base namespace.

Moves ReplaceFirstSubstringAfterOffset. It also changes the find/replace arguments to be string pieces (normally these are constants).

TBR=gauravsh@chromium.org for chromeos/network

Review URL: https://codereview.chromium.org/1200393002

Cr-Original-Commit-Position: refs/heads/master@{#335999}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e6dae46fc681a8ad62f6937a6ac1cab3d51938a9