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.  Setup: Code coverage is only supported for Debug-iphonesimulator builds.
+    (gtest_include_ios_coverage, ENABLE_TEST_CODE_COVERAGE?)
+
+1.  Run a test target that supports code coverage, for example
+    `ios_showcase_egtests`. Targets that support test target call
+    `coverage_util::SetupIfNecessary()`.
+
+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/Debug-iphonesimulator/coverage.profdata
+    path/to/coverage.profraw`
+
+1.  To see the **line coverage** for *all the instrumented source files*:

[lindsayw, 2017/04/03 15:32:09]

Sorry if this is obvious, but why do all of these points start with #1?

[lpromero, 2017/04/04 14:49:50]

Markdown rendering changes it to the required number when displaying. It's
useful in lists, as that way you can insert new list items without having to
shift the numbers.
There is an example at https://guides.github.com/features/mastering-markdown/.

+
+    `xcrun llvm-cov show
+    out/Debug-iphonesimulator/ios_showcase_egtests.app/ios_showcase_egtests
+    -instr-profile=path/to/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/showcase/core/app_delegate.mm`):
+
+    `xcrun llvm-cov show
+    out/Debug-iphonesimulator/ios_showcase_egtests.app/ios_showcase_egtests
+    -instr-profile=path/to/coverage.profdata -arch=x86_64
+    ios/showcase/core/app_delegate.mm`
+
+    ![](images/llvm-cov_show_file.png)
+
+    To see a **complete report**:
+
+    `xcrun llvm-cov report
+    out/Debug-iphonesimulator/ios_showcase_egtests.app/ios_showcase_egtests
+    -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/showcase/core` folder):
+
+    `xcrun llvm-cov show
+    out/Debug-iphonesimulator/ios_showcase_egtests.app/ios_showcase_egtests
+    -instr-profile=path/to/coverage.profdata -arch=x86_64 ios/showcase/core`
+
+    ![](images/llvm-cov_report_folder.png)