fix(form-field): remove 200px width since it messes up flex layouts (#7083)

* remove 200px width on form-field since it messes up flex layouts

* fix select expanding to 100%

src/lib/form-field/_form-field-theme.scss

@@ -23,12 +23,6 @@
   $underline-color-warn: mat-color($warn);
   $underline-focused-color: mat-color($primary);
 
-  // Define the width here so that it is easier for users to override with one css class worth
-  // of specificity.
-  .mat-form-field {
-    width: 200px;
-  }
-
   .mat-form-field-placeholder {
     color: $placeholder-color;
   }

src/lib/form-field/form-field.scss

@@ -6,6 +6,10 @@
 $mat-form-field-hint-min-space: 1em !default;
 // The height of the underline.
 $mat-form-field-underline-height: 1px !default;
+// Infix stretches to fit the container, but naturally wants to be this wide. We set this in order
+// to have a a consistent natural size for the various types of controls that can go in a form
+// field.
+$mat-form-field-default-infix-width: 180px !default;
 
 
 .mat-form-field {
@@ -60,6 +64,7 @@ $mat-form-field-underline-height: 1px !default;
   position: relative;
   flex: auto;
   min-width: 0;
+  width: $mat-form-field-default-infix-width;
 }
 
 // Used to hide the placeholder overflow on IE, since IE doesn't take transform into account when