Symlink debug folder instead of symlinking individual files

The original behavior of build_debug_applications.py was to traverse all
files recursively in the front_end folder and symlink each individual
file to the version in third_party/blink/renderer/devtools/front_end.

However, this requires rebuilding of Chrome when new files are added to
the application. After this CL, the developer experience is more
What-You-See-Is-What-You-Get.

This CL is also in preparation to remove the logic in C++-land to remove
the need for a separate debug folder. A follow-up CL will change the
behavior of `debug_devtools` to symlink directly into
out/[NAME]/resources/inspector, rather than an explicit sub-directory.

As a nice side-effect of this change, we are no longer symlinking the
debug folder for every version of the application. Previously, it would
iterate through all `application_names` and symlink the full debug
folder. This is time-consuming and did not provide any actual benefit.
Since we now symlink directly to the local version, we no longer need to
build the HTML files directly.

Bug: 986365
Change-Id: I7e54ac3af57e0a3941b88a46b71b3ddacd5664f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713344
Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@google.com>
Cr-Commit-Position: refs/heads/master@{#680518}