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This converts PaintTiming and ScriptLoader metrics to use the macro
helper, and WorkerThread termination metrics to use the function helper.
The distinction between function vs macro was made based on a guess on
how performance sensitive the code recording the metric appears to be.
In addition, the recorded enumerations have been updated to use the
kMaxValue convention, allowing:

- clang to enforce kMaxValue correctness
- autodeduction of the max value by the enum histogram macro/function
  helpers.

Bug: 742517, 1047547
Change-Id: I24f51cde75880c6ac42675724bc0fad321bcaf33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524933Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826737}