[code coverage] Allow turning off coverage with one config change

This CL removes the dependency between use_clang_coverage and
coverage_instrumentation_input_file so that the coverage feature can
be turned off with a single change by setting use_clang_coverage to
false.

Change-Id: Iff4d4c3207fa82d81a5b6bcfe417cc955764096d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698670
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#676959}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9eb53f51810a4b835020760d31e5f31601d1889e

config/coverage/OWNERS

@@ -1,3 +1,4 @@
 inferno@chromium.org
+liaoyuke@chromium.org
 mmoroz@chromium.org
 ochang@chromium.org

config/coverage/coverage.gni

@@ -19,11 +19,10 @@ declare_args() {
   # the file consists of multiple lines where each line represents a path to a
   # source file, and the paths must be relative to the root build directory.
   # e.g. ../../base/task/post_task.cc for build directory 'out/Release'.
+  #
+  # NOTE that this arg will be non-op if use_clang_coverage is false.
   coverage_instrumentation_input_file = ""
 }
 
 assert(!use_clang_coverage || is_clang,
        "Clang Source-based Code Coverage requires clang.")
-
-assert(coverage_instrumentation_input_file == "" || use_clang_coverage,
-       "Instrument a subset of source files requires enabling clang coverage.")