Add jdk 15 to third_party

Contents of README.google:
Name: Java Development Kit (JDK)
URL: https://jdk.java.net/15/
Version: 15
Revision: NA
License: the GNU General Public License, version 2, with the Classpath Exception.

Description:
This is pulled directly from the website for the JDK releases that has an open source license


Bug: 169692487
Change-Id: I2f95694ed8b09fd7a4a74697b0d56ca3aef8d393
diff --git a/.gitignore b/.gitignore
index bbe6eea..bee945e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -104,6 +104,12 @@
 third_party/nest/*
 third_party/openjdk/desugar_jdk_libs
 third_party/openjdk/desugar_jdk_libs.tar.gz
+third_party/openjdk/jdk-15/linux
+third_party/openjdk/jdk-15/linux.tar.gz
+third_party/openjdk/jdk-15/osx
+third_party/openjdk/jdk-15/osx.tar.gz
+third_party/openjdk/jdk-15/windows
+third_party/openjdk/jdk-15/windows.tar.gz
 third_party/openjdk/jdk-11-test
 third_party/openjdk/jdk-11-test.tar.gz
 third_party/openjdk/jdk-11/Linux
diff --git a/build.gradle b/build.gradle
index 981dcfd..c4e05ea 100644
--- a/build.gradle
+++ b/build.gradle
@@ -362,16 +362,19 @@
         linux: [
                 "third_party": ["openjdk/openjdk-9.0.4/linux",
                                 "openjdk/jdk8/linux-x86",
-                                "openjdk/jdk-11/Linux"],
+                                "openjdk/jdk-11/Linux",
+                                "openjdk/jdk-15/linux"],
         ],
         osx: [
                 "third_party": ["openjdk/openjdk-9.0.4/osx",
                                 "openjdk/jdk8/darwin-x86",
-                                "openjdk/jdk-11/Mac"],
+                                "openjdk/jdk-11/Mac",
+                                "openjdk/jdk-15/osx"],
         ],
         windows: [
                 "third_party": ["openjdk/openjdk-9.0.4/windows",
-                                "openjdk/jdk-11/Windows"],
+                                "openjdk/jdk-11/Windows",
+                                "openjdk/jdk-15/windows"],
         ],
 ]
 
diff --git a/third_party/openjdk/jdk-15/linux.tar.gz.sha1 b/third_party/openjdk/jdk-15/linux.tar.gz.sha1
new file mode 100644
index 0000000..2ac250c
--- /dev/null
+++ b/third_party/openjdk/jdk-15/linux.tar.gz.sha1
@@ -0,0 +1 @@
+116e48c5fcecdeccd6e26c07062298cdc3121ed7
\ No newline at end of file
diff --git a/third_party/openjdk/jdk-15/osx.tar.gz.sha1 b/third_party/openjdk/jdk-15/osx.tar.gz.sha1
new file mode 100644
index 0000000..0393063
--- /dev/null
+++ b/third_party/openjdk/jdk-15/osx.tar.gz.sha1
@@ -0,0 +1 @@
+ca8935fb824a4d79a987396b3f268f0ecb1bd51b
\ No newline at end of file
diff --git a/third_party/openjdk/jdk-15/windows.tar.gz.sha1 b/third_party/openjdk/jdk-15/windows.tar.gz.sha1
new file mode 100644
index 0000000..6f3c8dc
--- /dev/null
+++ b/third_party/openjdk/jdk-15/windows.tar.gz.sha1
@@ -0,0 +1 @@
+b0c7e22d73b9c5751ef8b62f8a9b10afe8aafa61
\ No newline at end of file