[Cronet] Enable Brotli

- removes the compile time flag to disable brotli when building Cronet.
- adds an API to enable Brotli on CronetEngine.
- adds three basic java tests in BrotliTest.java

BUG=710633
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2805053005
Cr-Commit-Position: refs/heads/master@{#465216}
diff --git a/components/cronet/url_request_context_config.h b/components/cronet/url_request_context_config.h
index 69e1b80..ce398236 100644
--- a/components/cronet/url_request_context_config.h
+++ b/components/cronet/url_request_context_config.h
@@ -87,6 +87,8 @@
       bool enable_spdy,
       // Enable SDCH.
       bool enable_sdch,
+      // Enable Brotli.
+      bool enable_brotli,
       // Type of http cache.
       HttpCacheType http_cache,
       // Max size of http cache in bytes.
@@ -132,6 +134,8 @@
   const bool enable_spdy;
   // Enable SDCH.
   const bool enable_sdch;
+  // Enable Brotli.
+  const bool enable_brotli;
   // Type of http cache.
   const HttpCacheType http_cache;
   // Max size of http cache in bytes.
@@ -201,6 +205,8 @@
   bool enable_spdy = true;
   // Enable SDCH.
   bool enable_sdch = false;
+  // Enable Brotli.
+  bool enable_brotli = false;
   // Type of http cache.
   URLRequestContextConfig::HttpCacheType http_cache =
       URLRequestContextConfig::DISABLED;