[PDF Viewer] Clean up PdfPluginAttributes usage

Clean up PdfPluginAttributes usage by:
1. Nesting the PdfPluginAttributes variable so it's only created when
   used.
2. Removing an unnecessary check that
   chrome.mimeHandlerPrivate.setPdfPluginAttributes is non-null. If
   chrome.mimeHandlerPrivate is non-null, then it should always be
   non-null.
3. Adding comments specifying that Print Preview doesn't use
   chrome.mimeHandlerPrivate.

Low-Coverage-Reason: TRIVIAL_CHANGE Minor code changes that shouldn't
have a major impact in code coverage.

Change-Id: I8d3289ec9f607729b757bac39f0e19016ae6d8ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4938386
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1209734}

chrome/browser/resources/pdf/pdf_viewer_base.ts

@@ -117,15 +117,16 @@ export abstract class PdfViewerBaseElement extends PolymerElement {
       plugin.toggleAttribute('pdf-viewer-update-enabled', true);
     }
 
-    // Pass the attributes for loading PDF plugin through the
-    // `mimeHandlerPrivate` API.
-    const attributesForLoading:
-        chrome.mimeHandlerPrivate.PdfPluginAttributes = {
-      backgroundColor: this.getBackgroundColor(),
-      allowJavascript: javascript === 'allow',
-    };
-    if (chrome.mimeHandlerPrivate &&
-        chrome.mimeHandlerPrivate.setPdfPluginAttributes) {
+    // PDF viewer only, as Print Preview doesn't use
+    // `chrome.mimeHandlerPrivate`.
+    if (chrome.mimeHandlerPrivate) {
+      // Pass the attributes for loading PDF plugin through the
+      // `mimeHandlerPrivate` API.
+      const attributesForLoading:
+          chrome.mimeHandlerPrivate.PdfPluginAttributes = {
+        backgroundColor: this.getBackgroundColor(),
+        allowJavascript: javascript === 'allow',
+      };
       chrome.mimeHandlerPrivate.setPdfPluginAttributes(attributesForLoading);
     }