string_util: Remove StartsWithASCII

This was used in exactly one location, FingerState::FlagsString, and can
easily be replaced by a call to std::string::rfind. Also add a new test
case to verify the change.

BUG=b:245989146
TEST=cros_sdk env FEATURES="test" emerge-${BOARD} chromeos-base/gestures

Change-Id: I832dd9a7cb61b3a4741d2a3439b4884746274f3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/gestures/+/5318731
Commit-Queue: Harry Cutts <hcutts@chromium.org>
Tested-by: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
Code-Coverage: Zoss <zoss-cl-coverage@prod.google.com>
Reviewed-by: Henry Barnor <hbarnor@chromium.org>
5 files changed