Add CLion setup documentation.

CLion is an IDE. This documentation is to make it easier to setup chromium development with CLion.

Change-Id: I020ec78e41aee54e425c40ae1ab4ee6f3cee27f9
Reviewed-on: https://chromium-review.googlesource.com/1110454
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569804}

docs/clion_dev.md

+# CLion Dev
+
+CLion is an IDE
+
+Prerequisite:
+[Checking out and building the chromium code base](README.md#Checking-Out-and-Building)
+
+[TOC]
+
+## Setting up CLion
+
+1. Install CLion 
+                
+1. Authenticate License
+    - https://g3doc.corp.google.com/devtools/ide/intellij/g3doc/docs/license-server.md?cl=head            
+
+1. Run CLion
+
+1. Increase CLion's memory allocation
+    - This step will help performance with large projects
+    1. Option 1
+        1. At the startup dialogue, in the bottom right corner, click `configure`
+        1. Setup `Edit Custom VM Options`:
+            ```
+            -Xss2m
+            -Xms1g
+            -Xmx5g
+            ```
+        1. Setup `Edit Custom Properties`:
+            ```
+            idea.max.intellisense.filesize=12500
+            ```
+    1. Option 2; 2017 and prior versions may not include the options to setup your `VM Options` and `Properties` in the `configure` menu. Instead:
+        1. `Create New Project`
+        1. `Help` > `Edit Custom VM Options`
+        1. `Help` > `Edit Custom Properties`
+
+## Chromium in CLion
+
+1. Import project
+    - At the startup dialogue, select `Import Project` and select your chromium directory
+
+1. Modify the `CMakeList.txt` file
+    1. Open the `CMakeList.txt` file
+    1. Add the following to the top
+        ```
+        set(CMAKE_BUILD_TYPE Debug)
+        include_directories(${CMAKE_CURRENT_SOURCE_DIR}/src)
+        ```
+    1. Remove any other `include_directories` the file contains
+        - the head should look like
+            ```
+            cmake_minimum_required(VERSION 3.10)
+            project(chromium)
+
+            set(CMAKE_CXX_STANDARD 11)
+
+            set(CMAKE_BUILD_TYPE Debug)
+
+            include_directories(${CMAKE_CURRENT_SOURCE_DIR}/src)
+
+            add_executable(chromium
+                ...)
+            ```
+
+## Building, Running, and Debugging within CLion
+
+1. `Run` > `Edit Configurations`
+1.  Click `+` in the top left and select `Application`
+1. Setup:
+    ```
+    Target: All targets
+    Executable: src/out/Defaults/chrome
+    Program arguments (optional): --disable-seccomp-sandbox --single-process
+    Working directory: .../chromium/src/out/Default
+    ```
+1. Click `+` next to the `Before launch` section and select `Run External tool`
+1. In the dialog that appears, click `+` and setup:
+    ```
+    Program: .../depot_tools/ninja
+    Arguments: -C out/Default -j 1000 chrome
+    Working directory: .../chromium/src
+    ```
+1. Click `OK` to close all three dialogs
+1. `Run` > `Run` or `Run` > `Debug`
+
+## Note on installing CLion on Linux
+
+For some reason, when installing 2018.1 through a package manager, it did not create a desktop entry when I tried it. If you experience this as well:
+
+1. Option 1
+    1. `cd /usr/share/applications/`
+    1. `touch clion-2018-1.desktop`
+    1. open `clion-2018-1.desktop` and insert:
+        ```
+        [Desktop Entry]
+        Name=CLion 2018.1
+        Exec=/opt/clion-2018.1/bin/clion.sh %u
+        Icon=/opt/clion-2018.1/bin/clion.svg
+        Terminal=false
+        Type=Application
+        Categories=Development;IDE;Java;
+        StartupWMClass=jetbrains-clion
+        X-Desktop-File-Install-Version=0.23
+        ```
+1. Option 2
+    1. Run CLion through the terminal `/opt/clion-2018.1/bin/clion.sh`
+    1. At the startup dialogue, in the bottom right corner, click `configure`
+    1. Click `Create Desktop Entry`
+    
+## Optional Performance Steps
+
+### Mark directories as `Library Files`
+
+To speed up CLion, you may optionally mark directories such as `src/third_party` as `Library Files`
+1. Open the `Project` navigation (default `Alt 1`)
+1. Right click the directory > `Mark directory as` > `Library Files`
+1. See `https://blog.jetbrains.com/clion/2015/12/mark-dir-as/` for more details
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