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This change makes boilerplate.py trim a leading ".\" (Windows's
current-directory prefix) or "./" (Unix's current-directory prefix)
from the filename argument, if present.

This is a convenience change for cases where the current-directory
prefix is prepended. For example, Windows PowerShell automatically
prepends ".\" to a path when tab completion is used (e.g., typing
"chrome/b" and pressing tab results in ".\chrome\browser"). This
change will remove artifacts of this for users who use tab completion
while typing the filename argument.

Currently:
`tools/boilerplate.py .\test.h` produces include guard `__TEST_H_`
`tools/boilerplate.py ./test.cc` or `.mm` produces `#include "./test.h"`

With this change:
`tools/boilerplate.py .\test.h` produces include guard `TEST_H_`
`tools/boilerplate.py ./test.cc` or `.mm` produces `#include "test.h"`

Change-Id: I19fd2e72c58135b5e87d4a1e22ec8bc76ef326aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157876
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761085}