Add tools for code coverage support in iOS.

docs/ios/coverage.md

Index: docs/ios/coverage.md
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+# Generate code coverage data
+
+1.  Build a test target for coverage:
+
+    ```
+    ninja -C out/Coverage-iphonesimulator ios_chrome_unittests
+    ```
+
+    Targets that support code coverage need to call
+    `coverage_util::ConfigureCoverageReportPath()`.
+    If you don't use `setup-gn.py`, you can set the gn argument
+    `ios_enable_coverage` to `true`.
+
+1.  Run the test target. If using Xcode, don't forget to set `Coverage` in the
+    target's scheme:
+
+    ![](images/coverage_xcode.png)
+
+1.  Find the `coverage.profraw` in the `Documents` folder of the app. You can
+    look in the console output of the instrumented target. For example:
+
+    ```
+    Coverage data at /Users/johndoe/Library/Developer/CoreSimulator/Devices/
+    82D642FA-FC18-4EDB-AFE0-A17454804BE4/data/Containers/Data/Application/
+    E6B2B898-CE13-4958-93F3-E8B500446381/Documents/coverage.profraw
+    ```
+
+1.  Create a `coverage.profdata` file out of the `coverage.profraw` file:
+
+    ```
+    xcrun llvm-profdata merge \
+        -o out/Coverage-iphonesimulator/coverage.profdata \
+        path/to/coverage.profraw
+    ```
+
+1.  To see the **line coverage** for *all the instrumented source files*:
+
+    ```
+    xcrun llvm-cov show \
+        out/Coverage-iphonesimulator/ios_chrome_unittests.app/ios_chrome_unittests \
+        -instr-profile=out/Coverage-iphonesimulator/coverage.profdata \
+        -arch=x86_64
+    ```
+
+    ![](images/llvm-cov_show.png)
+
+    To see the **line coverage** for a *specific instrumented source
+    file/folder* (e.g.
+    `ios/chrome/browser/ui/coordinators/browser_coordinator.mm`):
+
+    ```
+    xcrun llvm-cov show \
+        out/Coverage-iphonesimulator/ios_chrome_unittests.app/ios_chrome_unittests \
+        -instr-profile=out/Coverage-iphonesimulator/coverage.profdata \
+        -arch=x86_64 ios/chrome/browser/ui/coordinators/browser_coordinator.mm
+    ```
+
+    ![](images/llvm-cov_show_file.png)
+
+    To see a **complete report**:
+
+    ```
+    xcrun llvm-cov report \
+        out/Coverage-iphonesimulator/ios_chrome_unittests.app/ios_chrome_unittests \
+        -instr-profile=path/to/coverage.profdata -arch=x86_64
+    ```
+
+    ![](images/llvm-cov_report.png)
+
+    To see a **report** for a *folder/file* (e.g. `ios/chrome/browser` folder):
+
+    ```
+    xcrun llvm-cov show \
+        out/Coverage-iphonesimulator/ios_chrome_unittests.app/ios_chrome_unittests \
+        -instr-profile=path/to/coverage.profdata -arch=x86_64 ios/chrome/browser
+    ```
+
+    ![](images/llvm-cov_report_folder.png)