[code coverage] Set partial instrumentation iOS coverage try builder.

This is to only instrument CL affected files for iOS coverage try
builder for efficiency.

Bug: 1090188
Change-Id: I386dcafef9ee174a7b22cbc0073ee58118f4ed6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227305Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774432}

tools/mb/mb_config.pyl

@@ -914,7 +914,7 @@
       'ios-simulator': 'ios_simulator_debug_static_rbe_bot',
       'ios-simulator-full-configs': 'ios_simulator_debug_static_bot',
       'ios-simulator-cronet': 'ios_cronet',
-      'ios-simulator-code-coverage': 'clang_code_coverage_ios',
+      'ios-simulator-code-coverage': 'clang_code_coverage_ios_partial_instrumentation',
       'ios-simulator-cr-recipe': 'ios_simulator_debug_static_bot',
       'ios-simulator-multi-window': 'ios_simulator_debug_static_bot_multi_window',
       'mac-osxbeta-rel': 'gpu_tests_release_trybot_deterministic_mac',
@@ -1530,6 +1530,10 @@
       'use_clang_coverage', 'debug_static_bot', 'x64', 'ios', 'ios_use_goma_rbe',
     ],
 
+    'clang_code_coverage_ios_partial_instrumentation': [
+      'use_clang_coverage', 'debug_static_bot', 'x64', 'ios', 'ios_use_goma_rbe', 'partial_code_coverage_instrumentation',
+    ],
+
     'clang_tot_asan_lsan_static_release': [
       'clang_tot', 'asan', 'lsan', 'static', 'release',
     ],