[iOS][code coverage] Preserve coverage data in crashes.

Add %c to raw profile filename pattern so coverage data won't lost at
program crash. See:
https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#running-the-instrumented-program

Bug: 1090188
Change-Id: I4eab6137a385701186f2c330ce4be35e2694b229
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472920Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817265}

testing/coverage_util_ios.mm

@@ -28,8 +28,9 @@ void ConfigureCoverageReportPath() {
             .environment[@"SIMULATOR_SHARED_RESOURCES_DIRECTORY"];
     // UUID ensures that there won't be a conflict when multiple apps are
     // launched in one test suite in EG2. %m enables on-line profile merging.
-    NSString* file_name =
-        [NSString stringWithFormat:@"%@-%%m.profraw", NSUUID.UUID.UUIDString];
+    // %c helps preserve coverage data at crash.
+    NSString* file_name = [NSString
+        stringWithFormat:@"%@-%%m-%%c.profraw", NSUUID.UUID.UUIDString];
     NSString* file_path =
         [shared_resources_path stringByAppendingPathComponent:file_name];