[blinkpy] Measure unit test coverage

This change configures `run_blinkpy_tests.py` to measure the coverage of
meaningful files with Coverage.py. Coverage measurement is enabled for
the `blink_python_tests` script test target run on builders, but not for
`run_blinkpy_tests.py` itself when run locally by developers.

This change also upgrades some python2-style print statements so that
so that Coverage.py does not crash when importing those modules in
python3.

Change-Id: If452d8e10e3d8d157caa0cd5549f28ac82de1305
Bug: 1344709
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3759441
Commit-Queue: Jonathan Lee <[email protected]>
Reviewed-by: Weizhong Xia <[email protected]>
Reviewed-by: Thomas Anderson <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1026481}
diff --git a/BUILD.gn b/BUILD.gn
index a3cf0a49..4efccb33 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1573,9 +1573,12 @@
 
   script_test("blink_python_tests") {
     script = "//testing/scripts/run_isolated_script_test.py"
-    args = [ "@WrappedPath(" +
-             rebase_path("//third_party/blink/tools/run_blinkpy_tests.py",
-                         root_build_dir) + ")" ]
+    args = [
+      "@WrappedPath(" +
+          rebase_path("//third_party/blink/tools/run_blinkpy_tests.py",
+                      root_build_dir) + ")",
+      "--coverage",
+    ]
 
     data = [
       # These tests use //build/android/devil_chromium.py even when !is_android,