core: add support for iframe loading

[wchargin]

Summary:
We create a same-origin iframe and add to it a <script> element that
dynamically imports the plugin entry point and invokes its renderer.

For now, the plugin frame has no way to communicate with the host. In
the future, we’d like to be able to (e.g.) send “reload” signals from
host to plugin, and enable the plugin to query the host for global state
like the run selection.

Test Plan:
Cannibalize an existing plugin to provide an ES module entry point:

diff --git a/tensorboard/plugins/scalar/scalars_plugin.py b/tensorboard/plugins/scalar/scalars_plugin.py
index 1b864a74..6ba2b98b 100644
--- a/tensorboard/plugins/scalar/scalars_plugin.py
+++ b/tensorboard/plugins/scalar/scalars_plugin.py
@@ -24,6 +24,7 @@ from __future__ import print_function

 import collections
 import csv
+import textwrap

 import six
 from six import StringIO
@@ -62,6 +63,7 @@ class ScalarsPlugin(base_plugin.TBPlugin):
     return {
         '/scalars': self.scalars_route,
         '/tags': self.tags_route,
+        '/scalars.js': self._serve_module,
     }

   def is_active(self):
@@ -85,10 +87,28 @@ class ScalarsPlugin(base_plugin.TBPlugin):

   def frontend_metadata(self):
     return super(ScalarsPlugin, self).frontend_metadata()._replace(
-        element_name='tf-scalar-dashboard',
+        es_module_path='/scalars.js',  # DO NOT SUBMIT, obviously
         use_data_selector=True,
     )

+  @wrappers.Request.application
+  def _serve_module(self, request):
+    module = textwrap.dedent("""
+        console.log("scalars.js: loaded module under origin:", window.origin);
+        export function render() {
+          console.log("scalars.js: rendering");
+          document.body.innerHTML = `
+            <h1>meet the new scalars dashboard</h1>
+            <ul>
+              <li>loss: &#x1F4C9;</li>
+              <li>accuracy: &#x1F4C8;</li>
+            </ul>
+            <p>nice job, keep it up</p>
+          `;
+        }
+    """).strip()
+    return http_util.Respond(request, module, 'application/javascript')
+
   def index_impl(self):
     """Return {runName: {tagName: {displayName: ..., description: ...}}}."""
     if self._db_connection_provider:

Then verify that the plugin renders properly:

Also check that the reload button does not affect it (nor raise a
console error), and that switching among dashboards does not restamp the
iframe.

wchargin-branch: iframe-loading

[wchargin]

@stephanwlee: Does this approach seem reasonable to you?

[stephanwlee]

This is what I had in mind.

[wchargin]

Lovely. I’ll fix it up, then.