Add PPAPI test mock headers to *.isolate files.

This should have been part of https://codereview.chromium.org/1136493003.

BUG=419383

Review URL: https://codereview.chromium.org/1139913002

Cr-Commit-Position: refs/heads/master@{#329390}

chrome/browser_tests.isolate

@@ -67,7 +67,9 @@
           '../net/data/',
           '../net/tools/testserver/',
           '../ppapi/tests/test_case.html',
+          '../ppapi/tests/test_case.html.mock-http-headers',
           '../ppapi/tests/test_page.css',
+          '../ppapi/tests/test_page.css.mock-http-headers',
           '../testing/test_env.py',
           '../third_party/accessibility-audit/axs_testing.js',
           '../third_party/pyftpdlib/',
@@ -88,6 +90,7 @@
           '<(PRODUCT_DIR)/test_case.html.mock-http-headers',
           '<(PRODUCT_DIR)/test_data/',
           '<(PRODUCT_DIR)/test_page.css',
+          '<(PRODUCT_DIR)/test_page.css.mock-http-headers',
           '<(PRODUCT_DIR)/test_url_loader_data/',
           'browser/policy/test/asn1der.py',
           'browser/policy/test/policy_testserver.py',

chrome/interactive_ui_tests.isolate

@@ -48,7 +48,9 @@
           '../net/data/ssl/certificates/',
           '../net/tools/testserver/',
           '../ppapi/tests/test_case.html',
+          '../ppapi/tests/test_case.html.mock-http-headers',
           '../ppapi/tests/test_page.css',
+          '../ppapi/tests/test_page.css.mock-http-headers',
           '../testing/test_env.py',
           '../third_party/pyftpdlib/',
           '../third_party/pywebsocket/',