Improve _CheckUmaHistogramChanges presubmit

The _CheckUmaHistogramChanges checks whether newly added
UMA_HISTOGRAM_* calls only use defined histogram names.

This CL removes two ways to introduce false positives:
* Unrelated macro names which contain UMA_HISTOGRAM as a continuous
  substring
* More than one string literal on the line with the histogram name.

While those cases are rare, there seems no downside to fixing them.

Bug: 821981
Change-Id: Ie1a803f562883f567d577e742ed2ed87fd0dfe66
Reviewed-on: https://chromium-review.googlesource.com/978245
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545583}
2 files changed