GN: Add common header extensions
GN: Add common header extensions

GN now recognizes most common header file extensions:
*.h, *.hh, *.hpp, *.hxx

These are all used inside the chromium repository.
This change makes "gn check" more robust.

R=brettw@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/1933323002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#391056}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk